Add log level opt

In relation to https://github.com/rotundasoftware/parcelify/issues/10 this PR adds a `logLevel` opt to the parcelify factory function, and a `--loglevel` arg on the command line, for setting the npmlog logging level.


Is this necessary? I think there should only be one npmlog instance shared between `index.js` and `package.js` so it would work to just set `log.level` in `index.js`. Could be tripping...