Expose API to build protoc plugin with customization support

[neilisaac]

When working on a multi-language project, it's useful to use protoc plugins to keep the protobuf build system consistent across all generators. It would be nice to be able to compile a custom protoc-gen-rust using rust-protobuf with protobuf_codegen::Customize.

I accomplished this by patching protobuf_codegen to expose crate::compiler_plugin::plugin_main and crate::gen::all::gen_all in lib.rs, but you may want a more deliberate API.