Add flag to set --error-format from command line (#525)

This PR adds the ability for users to pass the flag `--@io_bazel_rules_rust//:error_format=json` in order to get machine readable output. The options match https://doc.rust-lang.org/rustc/command-line-arguments.html#option-error-format

This is most useful for IDE integrations that want to be able to parse compilation errors.