Enable vale for prose linting (#3)

* Enable vale in CI

* Add vale config

* Tweak vale

* Update config

* Update .gitignore

* Switch back

* Rename CI

* Add badges

* Tidy up CI

* Change image

* Configure vale

* Delint

* Update README

.github/styles/Google/DateFormat.yml

@@ -0,0 +1,9 @@
+extends: existence
+message: "Use 'July 31, 2016' format, not '%s'."
+link: 'https://developers.google.com/style/dates-times'
+ignorecase: true
+level: error
+nonword: true
+tokens:
+  - '\d{1,2}(?:\.|/)\d{1,2}(?:\.|/)\d{4}'
+  - '\d{1,2} (?:Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)|May|Jun(?:e)|Jul(?:y)|Aug(?:ust)|Sep(?:tember)?|Oct(?:ober)|Nov(?:ember)?|Dec(?:ember)?) \d{4}'