Add support for dot-seperate linters, improve linter tests

This PR first and formost implements support for dot-seperate filetypes,
a very trivial change.

This closes #132

But more importantly, this PR vastly improves the test quality for
`ale#linter#Get`. It enables us to reset the state of ale's internal
linter cache, to facilitate better testing, as well as making use of
mocked linters instead of depending on linters on disk (which may
change). In addition, a dummy linter is defined to test the autoloading
behavior.

Header guards were removed from all linters as:

* A: ale won't try and load linters if they already exist in memory
* B: we can't reset state for testing if they can't be loaded again

ale_linters/haskell/hlint.vim

@@ -1,12 +1,6 @@
 " Author: jparoz <jesse.paroz@gmail.com>
 " Description: hlint for Haskell files
 
-if exists('g:loaded_ale_linters_haskell_hlint')
-    finish
-endif
-
-let g:loaded_ale_linters_haskell_hlint = 1
-
 function! ale_linters#haskell#hlint#Handle(buffer, lines)
     let l:errors = json_decode(join(a:lines, ''))