Using milknake to build an sdist instead of a bdist_wheel

Thanks so much for creating tools for the Rust/Python ecosystem!

I was using [setuptools-rust](https://github.com/PyO3/setuptools-rust) and cffi to package some Rust code with Python. I tried to switch over to milksnake but ran into a problem (that might be inherent to milksnake or might be due to my misunderstanding how to use the library - not sure which).

Anyway, I can only distribute an sdist of my Python package because the underlying Rust code calls some underlying C code that uses SIMD instructions; so, in order to get full speed, I need users to compile natively on their own machine at the moment of "pip install ...". This is painful of course because Rust compile times are relatively slow, but the library runs ~50%+ faster after compiling to CPU-specific code; so hopefully the trade off is worth it.

So I got milksnake to work just fine when trying to build a bdist_wheel, but I couldn't figure out how to make it do a simple sdist that gets compiled on-the-fly so-to-speak when users pip install. Is this possible? If not, perhaps it's a possible feature addition.

Thanks again!