New Rule: Git Message Regex

[Jeehut]

A rule that allows to check git messages in specified branches (also defineable through regexes) using regexes. An example configuration could be something like this:

rules:
  - git_message_regex:
      branch: work/.*
      last_commits_count: 10
      matching: "\\A(?:\\[[^\\]]+\\] )?[A-Z](?:.){,49}(?:\\n\\n\\S.*)?\\z"

Whereas the branch value active could always only lint the active branch.

In combination with Overcommit (which could run ProjLint before committing) this rule would be even useful during development.