Added new functionality including song search functionality, added additional search parameters

[jogoe222]

• Added functionality for the song search feature (song_search and SongResult and a change in the link parsing to extract the song ID)
• Added additional search parameters for band_search and album_search such as release formats (there's a pull request for this by @pabe-github), additional notes etc.
• Integrated the "similar artists" feature from @msanlop's fork and pull request, including the README example
• Fixed the requests_cache.core import issue
• "Lyrical themes" has since been renamed to just "Themes"
• Added an other property to band_search results to allow access to additional search result parameters (issue by me https://github.com/lcharlick/python-metallum/issues/7)