Add sample to key and all three key targets

[AlexMitchellMus]

key= sets three opcodes to the same setting:

key=51

Is equivalent to writing:

pitch_keycenter=51
lokey=51
hikey=51

pitch_keycenter=sample is able to set the region to playback with the sample's own keycenter metadata.
Thus it would make complete sense to be able to also write:
key=sample

And also:
lokey=sample
hikey=sample

@paulfd mentioned the idea of key=sample: sfztools/sfizz#362 (comment)

These additional opcode's would work together with lock_ prefix suggestion: #13 and enable very clear logic, without adding complexity to SFZ format.