Add stdio option to process.run

[Nicell]

We need a way to inherit stdout/stderr with process.run to finish #171.

This adds support for default inherit and none options for stdio. Default buffers the output and returns stdout/stderr in the result table. Inherit uses the parent process stdout/stderr (needed for luthier). None just ignores stdout/stderr from the created process.

In the future it'd be great to:

• Add a forward stdio kind that both buffers the output and forwards it to the parent process stdout/stderr
• Support different kinds for stdout vs stderr and also support a string/buffer in for stdin