protoc-rust 3.0.0-alpha.1

protoc --rust_out=... available as API. protoc needs to be in $PATH, protoc-gen-run does not.
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# API to generate .rs files

API to generate `.rs` files to be used e. g. [from build.rs](https://github.com/stepancheg/rust-protobuf/blob/master/protobuf-test/build.rs).

Example code:

```rust
extern crate protoc_rust;

use protoc_rust::Customize;

fn main() {
    protoc_rust::Codegen::new()
        .out_dir("src/protos")
        .inputs(&["protos/a.proto", "protos/b.proto"])
        .include("protos")
        .run()
        .expect("protoc");
}
```

And in `Cargo.toml`:

```
[build-dependencies]
protoc-rust = "2.0"
```

Note 1: This API requires `protoc` command present in `$PATH`.
Although `protoc-gen-rust` command is not needed.

Note 2: Is advisable that `protoc-rust` build-dependecy version be the same as `protobuf` dependency. 

The alternative is to use
[pure-rust .proto parser and code generator](https://github.com/stepancheg/rust-protobuf/tree/master/protobuf-codegen-pure).