refactor sasslint linter (#2077)

Previous implementation required one to have sass-lint globally. This
allows you to have it locally, override the executable and add options.

autoload/ale/handlers/sasslint.vim

@@ -1,8 +0,0 @@
-" Author: KabbAmine - https://github.com/KabbAmine,
-"   Ben Falconer <ben@falconers.me.uk>
-
-function! ale#handlers#sasslint#GetCommand(buffer) abort
-    return ale#path#BufferCdString(a:buffer)
-    \   . ale#Escape('sass-lint')
-    \   . ' -v -q -f compact %t'
-endfunction