Support --no-optional flag

[jmandzik]

Useful for when npm modules have optional dependencies you don't want or don't work on your platform. For example, ldapjs had optional dependencies on some dtrace libraries that don't work on OSX and slow down installs. Being able to skip them like I can with npm would be badass.

[alexanderGugel]

This is something that we won't implement anytime soon. In understand that this is useful in certain scenarios though.