[FEATURE] Use Markdown for Release Notes

- The initial Markdown is automatically created from
  the commit logs. (These are only to be considered as a hints)
- Based on certain commit title prefixes, the script tries to
  determine, whether the commit is related to a fix or a feature
- Allowed prefixes are: NEW, FEATURE for features and FIX, PATCH for fixes
- Old commits didn't have these prefixes, in which case all commit logs are
  displayed and if they have a body they are assigned to features otherwise
  fixes.
- Release Notes can be previewed to check what they would look like on
  the website and in Sparkle.
- Markdown is much much more readable than JSON, so... good choice!
Print the executed comman if debug is set.
Merge branch 'master' of https://github.com/GPGTools/GPGTools_Core