Add new transposition function for parsed melodies

[amiika]

Currently there is undocumented :add parameter that increases or decreases the degree values, that works like:

zplay "0 1 2 3", add: 1 -> "1 2 3 4"

Problem with that is that it is done before/during parsing. Add new transposition function:

m = zparse "0 1 2 3 4"
transposition m, 3 -> "3 4 5 6"

[amiika]

Done in Ziffers 2