\chords bug with inversions and durations.

[Peter Chubb]

This short sample gives me syntax errors, although it matches the
syntax in the reference manual, and as far as I can tell, in parser.yy:

\score {
   \context ChordNames \chords {
        f2/c f4./+bes f8/a 
   }
}

Chords with duration and an inversion seem to be disallowed.