Define a label prefix that label-sync doesn't touch

[janpio]

Similar to #88, but not on a repository level but for a specific label prefix (which means we have more control):

It would be nice if we could define a label (e.g. topic/* or epic/*) that label-sync would skip and not touch when syncing. That would enable us to use it ad hoc in the repository to have more freestyle grouping, e.g. topic/foo after we encountered 5 issues about foo.

[maticzav]

Very nice!