Support C# linting with mono compiler mcs. (#250)

* Support netcore project linting.

* Support check on the fly.

* Remove debug.

* Rename csc.vim to mcs.vim as it should be.

* Update README.

* Update doc.

* Using `=~#` instead of `=~`.

doc/ale.txt

@@ -68,6 +68,7 @@ The following languages and tools are supported.
 * Bourne Shell: 'shell' (-n flag), 'shellcheck'
 * C: 'cppcheck', 'gcc', 'clang'
 * C++ (filetype cpp): 'cppcheck', 'gcc'
+* C#: 'mcs'
 * Chef: 'foodcritic'
 * CoffeeScript: 'coffee', 'coffelint'
 * CSS: 'csslint', 'stylelint'