Various improvements for the release preparation script

• Make the version optional
  • For --only-changelogs, we can take for granted rbm.conf has already been updated
  • We could add a parameter to bump only the build number
  • For all the other cases, we can check the number in rbm.conf, and increase the minor number
  • The version could be still specified, if wanted
• Add a --changelogs-only alias to --only-changelogs
• Use a fuzzy date parsing
  • We could use dateutil.parser: it's not standard library, but I think it's likely people already have it (I do). It won't be able to understand a date like 'tomorrow', but it will understand 'Aug 6'