Add support to watch external sources, such as GitHub repositories

[alfredbaudisch]

[alfredbaudisch]

I just found out that most of the work for this is already done in another Elixir project: meddle0x53/blogit/blob/master/lib/blogit/repository_providers/git.ex - it's a matter of adding those modules here and adjusting them to PardallMarkdown.

[rockchalkwushock]

@alfredbaudisch working on this now. Feel free to assign it to me.