Release 0.2.0

* Err results from public functions is now `Error`, not `String`.
  This is a breaking change in public api.
* PR #2: value interpolation (as separate values and in strings).
* Implement basic `@content` for mixins (some scoping issues remains).
* Support namespace properties.
* Improve parsing of unquoted strings.
* Implement the `call()` function.
* Implement the `ie-hex-str()` function.
* Allow alternative parameter names to `rgba()` function.
* Improve defered operators handling.
* Improve scopeing and value evaluating (but not enough).
* Allow function declarations in blocks.
* Implement `for`, `while`, and `each` in user-defined functions.
* Fix scoping error for while loops.
* Support css unary + and -.
* Misc internal or smaller improvements (some suggested by clippy).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rsass"
-version = "0.1.11"
+version = "0.2.0"
 authors = ["Rasmus Kaj <kaj@kth.se>"]
 categories = ["command-line-utilities", "web-programming"]
 keywords = ["scss", "sass", "css"]