Lastgenre: add classical music genre in the tree

[ghost]

[21:44:38] Hi, I see nothing about classical music in the genres-tree.yaml file (lastgenre plugin). Any reason why? Are you waiting for contributions?
[21:45:46] Seems like a reasonable addition; I just think it didn't show up in our scraper strategy.
[22:13:42] Ok, I will see if I can contribute. I don't know much about it
[22:50:33] SuperDupont: Feel free to ask if you need help to grok how the file is built up. Also be sure to look at genres.txt - that's the file for the actual whitelisted tags getting pulled from last.fm.

Based on genres.txt, here is a proposition with "classic" as the main genre and with these subgenres:
ballet
baroque music
concerto grosso
cantata
cantique
chamber music
classical music
concerto
gregorian chant
impressionist music
opera
oratorio
orchestra
organum
requiem
string quartet
symphony

Tags that do not appear in genres.txt, but that seem popular on last.fm:
baroque
classical
contemporary classical
modern classical
orchestral
sacred music
symphonic
They could be added to both genres.txt genres.yaml

[sampsyo]

Looks good. Could you please make eye change and open a pull request? To do so, you may need to figure out the right nesting for the canonicalization tree.