Glue for Vale (#522)

Add Vale linter definitions

ale_linters/markdown/vale.vim

@@ -0,0 +1,9 @@
+" Author: chew-z https://github.com/chew-z
+" Description: vale for Markdown files
+
+call ale#linter#Define('markdown', {
+\   'name': 'vale',
+\   'executable': 'vale',
+\   'command': 'vale --output=line %t',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
+\})