Make it easy to configure custom underlines

[njsmith]

Sphinx is picky about headline underlines: it requires a consistent hierarchy throughout your docs. So if you keep your changelog as part of your manual, it's important that the headlines towncrier generates be at the appropriate level to "fit in". (This is especially true when migrating an existing project to towncrier.)

Currently this is impossible. You do part of it by using a custom template, but (a) this requires copy/pasting the complex default template file into your project just to change two characters, which is unfriendly, and (b) the topline still uses a hardcoded === underline, and there's no way to configure this at all.

It would be nice if there were a configuration option like underlines = ["=", "-", "~"] that would let the user specify the desired headline underline hierarchy in a straightforward way.