Add colors for signify groups to our Zenburn

This really, *really* should be done up proper and submitted as a PR to the
@jnurmine/Zenburn repo.
Switch colorscheme to Zenburn
move solarized-specific highlighting syntax over into colors/solarized.vim

Arguably not solarized-specific, but...  This doesn't need to be in vimrc if
we're not using solarized, but I don't want to lose it, either.
Add after plugin to properly set our signify colorscheme tweaks

...why I'm needing to do it this way, I have no idea.
initial (trivial) todo snippets file
nit! README.md fixups