API to generate .rs files

API to generate .rs files to be used e. g. from build.rs.

Example code:

``` extern crate protocrustgrpc;

fn main() { protocrustgrpc::run(protocrustgrpc::Args { outdir: "src", includes: &["proto"], input: &["proto/aaa.proto", "proto/bbb.proto"], rustprotobuf: true, // also generate protobuf messages, not just services ..Default::default() }).expect("protoc-rust-grpc"); } ```

Note this API requires protoc command present in $PATH. Although protoc-gen-rust-grpc command is not needed.