feat: make custom branch patterns more useful

[nornagon]

This improves the existing support for the SUPPORTED_BRANCH_PATTERN
configuration variable.

1. The configured pattern is now consistently used everywhere we check
   branches. Previously there were a few places that hardcoded a check for
   Electron's branch pattern.
2. We now use the regex's groups to determine branch ordering, instead of
   always splitting by -.
3. Numeric groups are sorted numerically, otherwise groups are sorted
   lexically. This allows non-numeric branch patterns.
4. Documented the environment variables that control this behavior.
5. Changed a couple of error messages that referenced the default branch
   pattern to be more generic.
6. Added some tests for branch matching.

[jkleinsc]

LGTM