Made LilyPondCommand initializer explicit. CHANGE.

OLD: LilyPondCommand.__init__(*args).

NEW: LilyPondCommand.__init__(name, format_slot=None).

The new initializer is self-documenting. The old initializer was the opposite
of self-documenting.

CHANGE: you can no longer initialize one LilyPond command from another.
LilyPondCommand(command) no longer works. Use new() or copy.copy() instead.

This closes GitHub #307.