Run npm installing a child process

This completely skirts trying to fix the issue in npm where multiple
calls to load don't yield correctly configured instances of npm for use
through the programmatic API.

i.e. minit create can now be run multiple times in the same process
and correctly install the expected node_modules in each configured
location.

I have hopes that this will be fixed someday in npm so I haven't
completely changed the way we configure npm prior to executing the
command.

The exec promise/subprocess wrapper is provided by @stuk
Merge pull request #49 from mczepiel/child-process

Run npm installing a child process