Added support for goimports fixer (#1123)

* Added support for goimports fixer
* added test and executable check
* fixed test assertions to reflect executable check

autoload/ale/fixers/goimports.vim

@@ -0,0 +1,22 @@
+" Author: Jeff Willette <jrwillette88@gmail.com>
+" Description: Integration of goimports with ALE.
+
+call ale#Set('go_goimports_executable', 'goimports')
+call ale#Set('go_goimports_options', '')
+
+function! ale#fixers#goimports#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'go_goimports_executable')
+    let l:options = ale#Var(a:buffer, 'go_goimports_options')
+
+    if !executable(l:executable)
+        return 0
+    endif
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' -l -w'
+    \       . (empty(l:options) ? '' : ' ' . l:options)
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction