Theme updates

[jcollie]

Two patches:

• Allow loading themes from
  • absolute path
  • subdirectory of user's config directory
• Add a switch for showing a short preview of themes.

[mitchellh]

Awesome work. I did a refactor on how we search for themes and just pushed it up. I didn't test this at all. Can you take a look, run through it, and see if it works? And open to any feedback. My main goal was to simplify the loadTheme function a bit which was getting noisy, and to limit allocations in our happy path (at the expense of increasing allocations in our sad path).

I want to fully refactor out the themeDir function and enum in Config.zig to use the new one but I ran out of time tonight. Feel free to finish that if you want I think the direction I was going is pretty clear.