v0.6.1: webrpc-gen CLI v2

Splits target generator templates into their own repositories.

webrpc-gen now fetches the go, ts and js targets from remote git repositories:

https://github.com/webrpc/gen-golang v0.6.0
https://github.com/webrpc/gen-typescript v0.6.0
https://github.com/webrpc/gen-javascript v0.6.0

webrpc-gen is fully backward compatible with v0.6.0.

v0.6.0: webrpc-gen with built-in target templates (go, ts, js)

bump version to v0.6.0