Enhancement: allow piper to use any of the voices available in libritts

[ethereal-engineer]

By allowing template args for ${voice} for piper (program and server), piper can be used with any of the hundreds (or more) of voices included with libritts. There was an online tool to try them all out and I went through heaps of them to find one that I was keen on using. Can't think of it or find it right now, of course.