Expose node-elm-compiler processOpts

[farism]

Hello, first I wanted to say thank you for the recent update to the latest node-elm-compiler, which enabled this PR.

I was wanting a bit more granular control over the underlying elm-make process (specifically to be able to toggle stdio: 'ignore') so this PR exposes the node-elm-compiler processOpts option.

It's only available through the node API and not through the CLI, and I don't think that would be a commonly desired feature.

Please let me know if there are any questions. Thanks again!