Allow globbing in mainBranches

### Is your feature request related to a problem? Please describe.
At work, we have a quarterly release schedule. Sadly, this means I often have to maintain three or four release branches as well as master. Adding these release branches in the config with mainBranches makes this much nicer, but I have to do it for each one every time and manage which ones I care about.

### Describe the solution you'd like
It would be really nice to be able to specify a regex for the mainBranches. If it supported adding `release/*` then things would be greatly simplified.

### Describe alternatives you've considered
As mentioned, I _could_ just add each branch manually and remove them when appropriate.

### Additional context
I might try to spend some time and put out a PR for this.