Custom mirrors as an option

[Torxed]

Add option to support locally synced mirrors that aren't in the official mirror list (I for instance keep a local mirror for speed purposes, to push 1GB+ speeds in test to avoid downtime) which I can't currently use unless i build yet a separate ISO hard coded to it.

Would be neat to have a custom_mirror field some where, future work would allow for more than one. But one mirror is probably plenty fine for starters.

The idea was born out of issue #1.

[Torxed]

Slight bug left behind: It doesn't remove [core], [extra] and [community] for instance.
But the local mirror does get added. Will look in to what the best option here is, to completely remove the old ones (via option), or just re-arrange it and see if the local repo takes over.