Update exec to not leak INPUT env variables to child processes

[luketomlinson]

Resolves #309

By default, the child_process module in Node will use process.env if no environment is explicitly passed.

For security purposes, we are filtering out the INPUT_* environment variables to prevent these from being passed to subprocesses by default. We do this by first filtering process.env and explicitly passing it to the spawn method.

[luketomlinson]

Closing, since we have close #309