Add optional call to action field, with a configurable fallback

[sockdrawermoney]

At the conclusion of a blog post, it's sometimes nice to include a call to action. ("If you liked this, follow me on twitter" or "You can buy my book here" or "Please donate to..." etc.) See this issue, for example.

Here's how I envision this working:

There would be an optional single-line cta field in the front-matter.

If that cta was there, the contents would be attached to the post as a markdown formatted line.

If the cta wasn't there and there was a pre-configured fallback in the bumbleConfig, it would fall back to using that.

If you just put cta: false then it would not use any cta.

[sockdrawermoney]

Completed by #63