Add documentation support for .vale.ini and yml rule files.

This implements #57 and adds support for showing documentation when configuring vale (both in rule files and .vale.ini files).

It also introduces a rudimentary .ini-file parser that can potentially come in handy for #58.

The documentation folder from vale-ls was incomplete and I added a couple of things which I hope to merge back to vale-ls as well:

https://github.com/vale-cli/vale-ls/pull/29