Add base16 Penumbra themes

[zacharyweiss]

Howdy!

This PR adds Neal McKee's fantastic Penumbra theme. Two things I want input on prior to merge:

1. Credit: Neal welcomes ports and the theme is MIT licensed. In this repo, it appears as though the "author" field is meant to indicate "maintainer" (/ "author of the yaml"), hence listing myself, but I don't wish to claim credit for the palette itself. I've added the source to the README and per-theme descriptions; is there a more appropriate way to do this? @nealmckee: if you see this, any input / preferences?
2. Naming: upstream, the contrast variants are labeled "contrast+" and "contrast++". Your home repo and other PRs here seems to indicate the yaml name should match the slugified name field; I changed these variants to "Contrast Plus" and "Contrast Plus Plus" to conform. However this feels quite clunky/lengthy to me; any better alternative?

[JamyGolden]

Looks great