Feature Request - Add Disabled attribute to Song class (Checkmark in iTunes)

[brningpyre]

I use the checkmark in iTunes to organize a lot of my music. One of the main reasons I wanted to use libpytunes was to build a project to sync all my checked songs (10K+) to my Android. But this attribute doesn't seem to be recorded in the song object.

In the iTunes XML it's recorded for unchecked songs as:
<key>Disabled</key><true/>
And for checked songs, the Disabled key is simply missing (thanks, Apple).

Would it be possible to add this attribute?

[brningpyre]

Made a pull request for this:
#71