Optional exclusion of repositories in the Github widget

I don't necessarily want all of my Github repositories to appear on the sidebar (perhaps a work in progress I don't want to promote yet or the source repo a *.github.io page).

This PR will allow the user to exclude any repos from appearing in the sidebar using the `github_blacklist_repos` setting in the configuration file.

**Example:**
`github_blacklist_repos: "myblog.github.io,experimental,work-in-progress"` will exclude the three repositories with those names from appearing.

Absence of the `github_blacklist_repos` setting will indicate that nothing should be excluded, so this addition is backwards-compatible.