Ability to blacklist specific github repos from aside

I have a few boring repos like my github pages repo and my dotfiles repo, that I want to be able to blacklist in `_config.yml` from showing up in the github aside. They're almost always recently pushed, but aren't really interesting to showcase.

This patch allows an array of repo names to be specified in `_config.yml` under the optional `github_blacklist` key. Any repos that show up there will be omitted from the display.