Add Proselint (#185)

* Add Proselint
- Markdown
- Tex
- Text

* Use ale#handler#HandleUnixFormatAsWarning

* Indentation

ale_linters/text/proselint.vim

@@ -0,0 +1,8 @@
+" Author: poohzrn https://github.com/poohzrn
+" Description: proselint for text files
+
+call ale#linter#Define('text', {
+\   'name': 'proselint',
+\   'executable': 'proselint',
+\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\})