Add log level

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.


This is a bit weird in terms of code style, but it won't break any existing code.

Perhaps a more idiomatic approach would be to export a function which could take opts and return the Package constructor.