Issue/7/add support for all audio formats supported by rekordbox

[SimonZimmer]

Adds support for all audio formats that are supported by rekordbox. Also makes sure that the files are encoded using the same attributes for bit_rate, sample rate, codec and artwork found in the original file. Tested using two files from my rekordbox library that were dragged out of place, processed and put back manually in the library. Didn't encounter any issues 🙏

Additionally:

• Fix imports to make executing tests possible without calling setup install first
• Fix a bunch of cosmetics

Note: Although wave_alignment desperately needs a refactor, it wasn't done here to focus on the feature first.

[y-brehm]

@SimonZimmer I just realized that you need to have ffmpeg installed on your machine in order to run this. Could you please add this to the README?