Makefile/javascript: .js is now an extension Makefile knows about
configure/Javascript: add --enable-proto-js binding option

This will build Javascript bindings based on
https://github.com/dcodeIO/ProtoBuf.js/wiki

This needs the proto2js generator installed along PATH
build: generate Javascript bindings, dependencies

there's no install target yet; js output goes to machinetalk/generated-js
for now