Allow specifying extra files to install in Cargo.toml

[Keruspe]

Some software might want to install extra files like some sample configuration file or init systems integration.
That would be great to be able to specify that in Cargo.toml so that cargo install actually installs those.

[alexcrichton]

Thanks for the report! I think this is a dupe of #2729, though, so I'm going to close in favor of that.