FIX: use mix from the project root directory (#1954)

* FIX: use mix from the project root directory
* Move find root project function to autoloaded handlers
* add tests for #ale#handlers#elixr#FindMixProjectRoot

autoload/ale/handlers/elixir.vim

@@ -0,0 +1,13 @@
+" Author: Matteo Centenaro (bugant) - https://github.com/bugant
+"
+" Description: find the root directory for an elixir project that uses mix
+
+function! ale#handlers#elixir#FindMixProjectRoot(buffer) abort
+    let l:mix_file = ale#path#FindNearestFile(a:buffer, 'mix.exs')
+
+    if !empty(l:mix_file)
+      return fnamemodify(l:mix_file, ':p:h')
+    endif
+
+    return '.'
+endfunction