Haxe linter

Some (hopefully) helpful notes:
* [Haxe.org](https://haxe.org/)
* Compile-time error format: `{filename}.hx:{line num}: characters {from char}-{to char} : {message} : {specifier}` (specifier is not always there; see examples)
* Haxe compiler has several special compiler flags for linking, `-lib` and `-cp`. We might need to read those off `build.hxml` ([doc](http://old.haxe.org/doc/compiler))
* When targeting C#, Java or C++, it might have linking errors in a different format.
* You might want to also see how vaxe's make command works: [this](https://github.com/jdonaldson/vaxe/blob/master/autoload/vaxe.vim#L327-L369)

Compiler error samples (targeting Neko VM):
```
File.hx:5: characters 8-9 : Missing ;
File.hx:10: characters 4-5 : Unexpected }
File.hx:5: characters 8-9 : Unknown identifier : a
```

I'll try to implement this when I have some spare time.