Add dhall-format as a Fixer

https://github.com/dhall-lang/dhall-lang

test/fixers/test_dhall_fixer_callback.vader

@@ -0,0 +1,11 @@
+Before:
+  call ale#assert#SetUpFixerTest('dhall', 'dhall')
+
+After:
+  call ale#assert#TearDownFixerTest()
+
+Execute(The default command should be correct):
+  AssertFixer
+  \ { 'read_temporary_file': 1,
+  \   'command': ale#Escape('dhall') . ' format --inplace %t'
+  \ }