Added "H" key to turn off lyrics highlighting

I'm not convinced lyric highlighting should necessarily even be on for
all singalongs. This provides an out if you don't want it.

Two reasons:

First, I'm worried that it takes something away from relying on your
fellow persons to get the timing right.  In live-band Karaoke scenarios,
especially if the person switching chords is good at it, this could be a
huge boon.  For making videos, also a great help.  In scenarios where
you have unreliable devices, a potentially less-than-reliable chord
switcher, lack of click track, etc, it may be less confusing for
everyone involved if the system isn't occasionally prompting you to sing
when you shouldn't.

Second, phones are often laggy and with the screensaver off, they work
great as display devices.

A solution to this problem may be having the chord changes effect the
overall playback tempo less directly than they currently do, thus making
the system more fault tolerant, and end user devices able to specify a
"read-ahead" to better predict when to trigger timing information.
However, this takes away from the ability to quickly change tempo, half
the song and pick back up, etc. that is one thing I love about the
current system.

At any rate, now, the choice is yours.