Add engines field to package.json to document supported nodes

[doug-wade]

Addresses #4. Now, when users on older, unsupported nodes will get a warning that looks something like:

npm WARN engine [email protected]: wanted: {"node":">=6.0.0"} (current: {
"node":"4.2.1","npm":"2.14.7"})

[doug-wade]

This is a not-super-user-friendly resolution to this issue, since node 4 will be supported for some time. If we'd like, I can set up babelification to support older nodes without re-writing the existing code. I figured in the mean time, we can at least warn users of older nodes

[Rich-Harris]

Thanks! you're right, we need to ship a version that runs in Node 4 (and older browsers, for the REPL). Will open an issue on the main repo