Tree root, subcommands, inheritance issues

• Add support for subcommands [fix #32]
• Define metadata tree root [fix #26]
• Enable regular expressions in --filter [fix #35]
• Support merging dictionary values as well
• Build Python 3 package for pip as well
• Add more detailed logging for easier debugging
• Correctly handle deep inheritance [fix #31]
• Load all strings from YAML files as Unicode
• Prevent data modification in filter [fix #30]
• Fix inheritance of scattered files [fix #25]