feat(core): add external repos for package installs

[BamBalaam]

I'll admit, I had totally forgotten about having asked you questions in how I could implement this.

This works but has a slight bug, I wonder if you have some insight:

• If both repo and package are added at the same time it works fine
• If the repo is added, settings are reloaded, and then the package is added, it works fine
• If the package is added, settings are reloaded, Package Control will complain that it can't find the package (which is the expected behaviour). However, if I now add the repo to the list, when I reload Package Control will complain again about the package not being found. Only on the second reload will the package install correctly.

Any reason for this? Should I add the repos way earlier in the process?

Closes #39

[BamBalaam]

@dvcrn

(I had some issues with differing versions between my local version and what I was pushing. You can squash the commits if this ever gets merged If you're against squashing commits I can close the PR and reopen whenever we fix the bug mentioned above)