How to extend pithy or write a plugin?

BTW this this library is great!

I want to use pithy to generate SVG. The [builtin tags don't include SVG's `` tag](https://github.com/caolan/pithy/blob/f887990/lib/pithy.js#L20-L34). Instead of making pithy bloated, I want to write a plugin.

Start with lib/pithy.js, especially the built-in tag definitions linked in the issue, and inspect how tags are exposed or extended. Clarify whether the expected outcome is plugin documentation or an extension mechanism, then verify that the documented approach supports SVG's tag without changing the built-in tag set.

Written by the indexing model from the issue text.