plugin: Add 'rstcheck' for reStructuredText linting

Hi,

This MR adds 'rstcheck' as a plugin for linting reStructuredText files.

While it works, I can already understand the way it does so may not be optimal enough. That is because rstcheck doesn't support stdin inputs or pushing file content as an argument, which means files are linted after they are saved. One last problem it has is it is only able to find one issue at a time.

Btw, 'rstcheck' can be imported as a python library and file content can be linted by calling rstcheck.check(str) but importing it was not possible even though it can run as an executable.

Finally, I picked 'rstcheck' because the issue requesting the feature seem to prefer it. If another program is preferred, I can implement that instead.

Closes #1560 (closed).