Change default export to named one

Now you should import this plugin like this

```js
import { uglify } from 'rollup-plugin-uglify';

export default {
  ...
  plugins: [uglify()]
  ...
}
```

Added babel code frame so now syntax errors should look prettier
```
    > 1 | var = 1
        |    ^ Name expected

Error: Error transforming bundle with 'uglify' plugin: Name expected
```

Started testing on node 6, 8 and the latest one.

.babelrc

@@ -0,0 +1,5 @@
+{
+  "presets": [
+    ["@babel/env", { "targets": { "node": 6 } }]
+  ]
+}

.gitignore

@@ -1,4 +1 @@
-node_modules/
-coverage
-npm-debug.log
-dist
+node_modules

.travis.yml

@@ -1,3 +1,5 @@
 language: node_js
 node_js:
   - "node"
+  - "8"
+  - "6"

README.md

@@ -15,7 +15,7 @@ npm i rollup-plugin-uglify -D
 
 ```js
 import { rollup } from 'rollup';
-import uglify from 'rollup-plugin-uglify';
+import { uglify } from 'rollup-plugin-uglify';
 
 rollup({
     entry: 'main.js',

index.js

@@ -1,9 +1,7 @@
-const minify = require("uglify-js").minify;
+const { codeFrameColumns } = require("@babel/code-frame");
+const { minify } = require("uglify-js");
 
-function uglify(userOptions, minifier) {
-  if (minifier === undefined) {
-    minifier = minify;
-  }
+function uglify(userOptions, minifier = minify) {
   const options = Object.assign({ sourceMap: true }, userOptions);
 
   return {
@@ -12,11 +10,15 @@ function uglify(userOptions, minifier) {
     transformBundle(code) {
       const result = minifier(code, options);
       if (result.error) {
+        const { message, line, col: column } = result.error;
+        console.error(
+          codeFrameColumns(code, { start: { line, column } }, { message })
+        );
         throw result.error;
       }
       return result;
     }
   };
 }
 
-module.exports = uglify;
+exports.uglify = uglify;

package.json

@@ -23,11 +23,16 @@
   "author": "Bogdan Chadkin <trysound@yandex.ru>",
   "license": "MIT",
   "dependencies": {
+    "@babel/code-frame": "^7.0.0-beta.47",
     "uglify-js": "^3.3.25"
   },
   "devDependencies": {
-    "jest": "^22.1.4",
-    "prettier": "^1.10.2",
-    "rollup": "^0.54.1"
+    "@babel/core": "^7.0.0-beta.47",
+    "@babel/preset-env": "^7.0.0-beta.47",
+    "babel-core": "^7.0.0-bridge.0",
+    "babel-jest": "^22.4.4",
+    "jest": "^22.4.4",
+    "prettier": "^1.12.1",
+    "rollup": "^0.59.1"
   }
 }

test/fixtures/unminified.js

@@ -1,6 +1,6 @@
 "use strict";
-var a = 5;
+window.a = 5;
 
-if (a < 3) {
+if (window.a < 3) {
   console.log(4);
 }

test/test.js

@@ -1,16 +1,15 @@
 const assert = require("assert");
-const rollup = require("rollup").rollup;
-const readFile = require("fs").readFileSync;
-const uglify = require("../");
+const { rollup } = require("rollup");
+const { readFileSync: readFile } = require("fs");
+const { uglify } = require("../");
 
 test("minify", async () => {
   const bundle = await rollup({
     input: "test/fixtures/unminified.js",
     plugins: [uglify()]
   });
   const result = await bundle.generate({ format: "cjs" });
-  expect(Object.keys(result)).toHaveLength(2);
-  expect(result.code).toEqual('"use strict";var a=5;a<3&&console.log(4);\n');
+  expect(result.code).toEqual('"use strict";window.a=5,window.a<3&&console.log(4);\n');
   expect(result.map).toBeFalsy();
 });
 
@@ -23,7 +22,6 @@ test("minify via uglify options", async () => {
     banner: "/* package name */",
     format: "cjs"
   });
-  expect(Object.keys(result)).toHaveLength(2);
   expect(result.code).toEqual('/* package name */\n"use strict";\n');
   expect(result.map).toBeFalsy();
 });
@@ -34,7 +32,6 @@ test("minify with sourcemaps", async () => {
     plugins: [uglify()]
   });
   const result = await bundle.generate({ format: "cjs", sourcemap: true });
-  expect(Object.keys(result)).toHaveLength(2);
   expect(result.map).toBeTruthy();
 });
 
@@ -66,14 +63,15 @@ test("throw error on uglify fail", async () => {
     const bundle = await rollup({
       input: "test/fixtures/failed.js",
       plugins: [
-        uglify({}, () => ({
-          error: Error("some error")
-        }))
+        {
+          transformBundle: () => ({ code: "var = 1" })
+        },
+        uglify()
       ]
     });
     await bundle.generate({ format: "es" });
     expect(true).toBeFalsy();
-  } catch (err) {
-    expect(err.toString()).toMatch(/some error/);
+  } catch (error) {
+    expect(error.toString()).toMatch(/Name expected/);
   }
 });