Add support for --config flag

[anthonydandrea]

Cargo allows a --config <PATH> flag to merge a Cargo config.toml with the environment during a build. This is particularly helpful for cross-compilation projects which may manage their builds through different config files.

This allows the user to pass a --config /some/path to a config.toml to feed into the cargo build command

[RazrFalcon]

Thanks!

[anthonydandrea]

Thanks for the speedy review!

Any chance the latest could be published to crates.io sometime soon? Looks like it's been awhile, and that'd help me be able to use this change with my normal crates.io-based build system.

[RazrFalcon]

Will do on weekends.