Support regex or globs for MainBranches config

**Is your feature request related to a problem? Please describe.**
My organization's upstream branches change somewhat frequency as different ones are created for each major version of our product we release. It is tedious to update the lazygit configuration to add each new version with each new release.

**Describe the solution you'd like**
I would like to be able to express main branches in configuration via a glob pattern or regular expression.

**Describe alternatives you've considered**
A scheduled job to update the configuration file periodically - seems like overkill

**Additional context**
I would like to develop and submit this PR assuming the overall idea is considered okay to add to this project.