Auto merge of #2779 - matklad:propagete-color-config, r=alexcrichton

Propagate --color option to rustc

closes #2740

Will try to add a test for this soon (and fix failing tests if any, compiling/running tests locally is slow :( ).

I am not sure what is the right place to add `--color` option to the command line. I use [`build_base_args`]. [`process`] also looks like a good candidate, because it is more general, but if we look at the [`CommandType`] we see that only `rustc` command supports `--color`.

[`build_base_args`]: https://github.com/matklad/cargo/blob/1f7504397ce7c40ff708e2d31da164822e88ed37/src/cargo/ops/cargo_rustc/mod.rs#L449
[`process`]: https://github.com/matklad/cargo/blob/1f7504397ce7c40ff708e2d31da164822e88ed37/src/cargo/ops/cargo_rustc/mod.rs#L608
[`CommandType`]: https://github.com/matklad/cargo/blob/1f7504397ce7c40ff708e2d31da164822e88ed37/src/cargo/ops/cargo_rustc/engine.rs#L102