Added support for accordion tablatures

Hi, Paul,

This is the first release of the code with tablatures. I will try to improve the inference rules to get  a better initial tablature.

I dont know how the pull request will afect your code. You can see a working copy of my work at "http://flvani.github.io/diatonic-map/mapa.html". There, under the tab "songs", I use de abcjs to render a song and even edit it. If the song contains a clef  of type "accordionTab", I will infer the tablature. If the tablature exists,  it will be parsed.

Hope to hear from you.
Regards,
Flávio.