Lint mode?

[gforcada]

Hi! 👋 thanks for creating this tool, we have been using since a few weeks and it is so much nicer to work with it, than resolving git conflicts on CHANGES.rst 😄

There is one thing that would be nice to add to it though, as today is the second time I bump into the same problem: we have some custom sections newsfiles suffixes (breaking, feature and bugfix) but every now and then we write the newsfiles as .fix or .feat or such.

Would it be possible to add a --check option to towncrier that verifies that all newsfiles have a valid suffix? 🤔

I could even try to add it myself if it is considered a good idea.

[tjanez]

See #154.