Add support for exceptions in non default branch warning

[Urgau]

This PR adds support for specifying exceptions to the global default branch according to the PR title.

This is done to improve the usefulness of the non-default branch warning by making it warn if a PR trying to target the stable branch is actually targeting the master branch.

[assign]
warn_non_default_branch.enable = true

[[assign.warn_non_default_branch.exceptions]]
title = "[beta"
branch = "beta"

[[assign.warn_non_default_branch.exceptions]]
title = "[stable"
branch = "stable"

cc #1852 @cuviper
r? @ehuss

[Urgau]

We could also use a HashMap<String, WarnNonDefaultBranchException> where the string would be the branch name, which would permit this and be more consistent with the rest of the configs, but we would losz the "exceptions" field. I'm not sure which one is better.

[assign.warn_non_default_branch]
enable = true

[assign.warn_non_default_branch."beta"]
title = "[beta"

[assign.warn_non_default_branch."stable"]
title = "[stable"