Add support for getting a specific branch from a plugin repo

[smanilov]

Reopening #35.

It has been reverted, due to a build outage and insufficient testing.

The state of the art at the moment is https://stackoverflow.com/a/40189209/1864317. The downside is that the workaround disables plugin updates.

This feature is standard in other plugin managers.

[zwimer]

This would be useful as main things have switched to main.