Allow trill on chord

[ahankinson]

          I guess we also want a trill? Not very likely, but I am sure somebody will find one

Originally posted by @lpugin in #110 (comment)

[jenniferward]

Brahms has trills on chords, even double trills if I recall correctly (dyads, each of which are trilled), but these are mid-movement and not at the beginning. Just the same, I wouldn't rule it out for incipits.