Learn some -q/-s/-v flags for more advanced output handling

[jasonkarns]

The install command could be more versatile with -q/-s/-v flags for output handling. (leveraging npm's loglevel and perhaps separating default-packages' own output from npm)

[jasonkarns]

Also worth noting that at this point, the only non-error output we generate is from the install hook. https://github.com/nodenv/nodenv-default-packages/blob/master/etc/nodenv.d/install/default-packages.bash#L12

The install subcommand is silent except whatever npm emits.