Support for JSON Linting (#6)

* Fix Composer script 'phplint'

Fixed:
- Syntax for multiple starting points
- Match only regular PHP files

* Add Composer script 'jsonlint'

For linting metadata files.

Added:
- Composer package 'seld/jsonlint'
- Composer script 'jsonlint'

Changed:
- Execute 'jsonlint' in Travis

.travis.yml

@@ -28,6 +28,7 @@ install:
     - mkdir -p build/logs/
 
 script:
+    - composer jsonlint
     - composer phplint
     - composer phpcs
     - composer phpunit -- --coverage-clover "build/logs/clover.xml"

composer.json

@@ -28,6 +28,7 @@
         "phpunit/phpunit": "^5.7 || ^6.5",
         "squizlabs/php_codesniffer": "^3.0",
         "php-coveralls/php-coveralls": "^2.0",
+        "seld/jsonlint": "^1.7",
         "cache/void-adapter": "^0.3.1",
         "tedivm/stash": "~0.14",
         "pimple/pimple": "^3.0"
@@ -47,10 +48,12 @@
             "@tests"
         ],
         "tests": [
+            "@jsonlint",
             "@phplint",
             "@phpcs",
             "@phpunit"
         ],
+        "jsonlint": "find metadata tests -type f -name '*.json' -print0 | xargs -0 -n1 -P8 vendor/bin/jsonlint | grep -v '^Valid JSON'; test $? -eq 1",
         "phplint": "find src tests -type f -name '*.php' -print0 | xargs -0 -n1 -P8 php -l | grep -v '^No syntax errors detected'; test $? -eq 1",
         "phpcs": "php vendor/bin/phpcs -ps --colors src/ tests/",
         "phpcbf": "php vendor/bin/phpcbf -ps --colors src/ tests/",