Add a way to grammar check prose

[mbmjensen]

Ideally this could either work lint style, letting me run it over a selection of lines (or the whole file) and generate a quick fix or location list or it would function more like the built-in spell checker. I think the former is more interesting and would probably have more flexibility since it could use different cli solutions for grammar checking.

[mbmjensen]

One possibility: https://github.com/dpelle/vim-LanguageTool

[mbmjensen]

Probably limited use because unless I can find a grammar checker that runs strictly locally, I cannot use it for work