Add GitReader for generating changelog from commit history of GIT repos

[atufkas]

A new Reader implementation for GIT ("GitReader") should enable developers to "reversely" generate a changelog by fetching messages from a GIT commit log history, ideally by parsing tags as releases and their commits as log entries. At first place it should be possible

• fetch a specific commit history by passing the GIT repo url
• define a specific tag to parse (parse all by default)

Enhanced features could be:

• defining a tag range to be fetched and parsed
• a list of conversion rules for
  • setting defaults missing message (log entry) types
  • defining mappings for unknown message types
  • defining filter rules for messages to be skipped (regex...)

In the ideal world, a git commit history is clean, already in conventional commit log format and only consists of meaningful change sets and useful commit messages. However, the real world shows us that we'll always have a lot of noise (for good reason): Cleanups, merges, typo corrections are just a few examples.