CLI commands for demo

[alantrrs]

• [x] emp listen should launch the listen.js script
• [x] emp run experiment-name /path/ should run a experiment from the given path
• [x] emp run empiricalci/hello-world/sample/x35cg_d3 should fetch the experiment information from empiricalci.com and run it
• [ ] emp shows usage instructions & version
• [ ] emp versions returns version

[alantrrs]

Got started with the first two in here: https://github.com/alantrrs/emp/commit/5faf897b9707ca5b87e14595c3ec745d9e09bc87