Add shfmt fixer for sh files (#1083)

* Add shfmt fixer for sh files
* Add tests for shfmt fixer

autoload/ale/fixers/shfmt.vim

@@ -0,0 +1,17 @@
+scriptencoding utf-8
+" Author: Simon Bugert <simon.bugert@gmail.com>
+" Description: Fix sh files with shfmt.
+
+call ale#Set('sh_shfmt_executable', 'shfmt')
+call ale#Set('sh_shfmt_options', '')
+
+function! ale#fixers#shfmt#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'sh_shfmt_executable')
+    let l:options = ale#Var(a:buffer, 'sh_shfmt_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . (empty(l:options) ? '' : ' ' . l:options)
+    \}
+
+endfunction