Allow setting of a default prefix for headings.

[MysterAitch]

Fixes gh-56 / Closes #56

There is scope for overriding this per-heading (e.g. via the yml configuration) and adding a suffix (e.g. to allow for == header == style headings) but I figure let's start of with a smaller PR first 👍

[philwebb]

From #56 (comment)

Despite this, I cannot help but wonder if the idea of a prefix / heading level should be removed entirely, and just have it configured via the title itself - for example, to get level three headings:

That's certainly simpler, but I worry that it adds quite a bit of noise to the configuration. It would also break back-compatibility. I wonder if users want to mix heading levels in one output? We could look to see if the title starts with # and if it does we could skip applying the default?

[MysterAitch]

I wonder if users want to mix heading levels in one output?

I can see a case for this -- for example, having most headings being level two, and the different categories of issue (bug, fix, etc) being nested inside another heading at level three.

(Or three and four, or whatever...)