Make base_branch to accept branch patterns

[Cvelmurugan1989]

Currently base_branch is accepting only one string, but it would be useful if we have more than one branch name.

something like below pattern it should be accepting from user to cover the PRs from all branch names start with feaature, dev1 & master

base_branch: [master dev* feature*]

[techgaun]

I would like to take a stab at this one. @itsdalmo (tagging you based on contribution graph) unless you have any objection, we could just follow the above format. Alternatively, we could just use regex string to specify our base_branch configuration.