Add shfmt fixer for sh files (#1083)

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

test/fixers/test_shfmt_fixer_callback.vader

@@ -0,0 +1,24 @@
+Before:
+  Save g:ale_sh_shfmt_executable
+  Save g:ale_sh_shfmt_options
+
+  " Use an invalid global executable, so we don't match it.
+  let g:ale_sh_shfmt_executable = 'xxxinvalid'
+  let g:ale_sh_shfmt_options = ''
+
+Execute(The shfmt callback should return the correct default values):
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('xxxinvalid'),
+  \ },
+  \ ale#fixers#shfmt#Fix(bufnr(''))
+
+Execute(The shfmt callback should include custom shfmt options):
+  let g:ale_sh_shfmt_options = '--some-option'
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape(g:ale_sh_shfmt_executable)
+  \     . ' --some-option',
+  \ },
+  \ ale#fixers#shfmt#Fix(bufnr(''))