Allowed result code to be run via a child node process.

I find it's very useful to allow `esnext` to run the code it compiles, rather than just printing to stdout (or saving to a file).

Before this change, I was running `esnext file.js | node`, which would run the generated output.  This change essentially does that exact thing, but as a flag to esnext.  `esnext -e file.js` (or `esnxt --exec file.js`).