Allow back ups of specific repositories

[mj0nez]

I wanted the option to backup only a subset of my repositories. Therefore, the new, optional config is used alongside the existing discovery mechanisms. Before the backup starts, all results are merged, filtered for distinct repos and the exclude list is applied. This could be handy for people who contribute across many projects, have huge collection of starred repos or just want more control over their backup process.

I took the liberty to align the discovery mechanisms of both sources - I thought it made sense at this point.

[!IMPORTANT]
Requires #36: I've tried to keep both PRs separate but in the end it made no sense to replicate some work.