Proposal: create `renderer` package to enable vendoring

[radu-matei]

First of all, hi and thanks for contributing this cool program!

I've been looking at integrating a markdown renderer in one of our projects, and mdcat seems to contain the minimum functionality on top of blackfriday - however, in its current form, it is impossible to vendor mdcat since all code is contained in the main package.

This is a proposal to break out renderer.go into its own package that can then be vendored out by different projects.

Note that this would have no impact in the functionality of mdcat. I will also open a PR for this, but opened this issue to track all discussions for this subject.

Thanks!