Change Slash theme submodule repo to own forked version

It turns out that the Slash theme is missing a couple of things that I'd
like, so I'm going to fork it, add the things I want, and send pull
requests. Until my pull requests get merged, it looks like I'll be
maintaining my own forked version of the theme.

  https://github.com/lencioni/Octopress-Theme-Slash
Improve styles

While working on my first blog post, I noticed a few styles that I
wanted to tweak slightly. Most of the styles here make subtle
improvements to readability and design on top of the Slash theme. Some
future work could probably be done to try and get some of these styles
merged upstream, but in the meantime, this gets the job done.
Add "10 Easy Ways to Craft More Readable CSS"