Namespace the proto packages so we can run both v1 and v2 at the same time

Author: bep

Diff for: internal/embeddedsass/README.md

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+
+* Install protobuf: https://github.com/protocolbuffers/protobuf
+* Install the Go plugin: go get -u google.golang.org/protobuf/cmd/protoc-gen-go
+* protoc --go_opt=Membedded_sass_v1.proto=github.com/bep/godartsass/internal/embeddedsassv1 --go_opt=paths=source_relative --go_out=. embedded_sass_v1.proto