Add --no_pdf parameter

* Add the --no_pdf parameter to the cli options.
* Add a link to an example of achtelbass output
  on youtube.
* Add a file TODO.txt to the docs directory.
Add no_pdf parameter

* Add no_pdf parameter which introduces
  another dependency: musescore.
* Fix some opt arg typos.
Set default value of display_pdf to False
Add slider widget for setting tempo
Add locales entry for Tempo