Support using as a protoc plugin

[paralin]

ts-proto has the nice feature that it supports being called as a protoc plugin:

		--plugin=./node_modules/.bin/protoc-gen-ts-proto \
		--ts-proto_out=$$(pwd)/vendor \
		--ts-proto_opt=forceLong=long \
		--ts-proto_opt=oneof=unions

... supporting protoc-gen-ts-protons would also be good.