Support --no-optional (skip optional dependencies)

[Tal500]

I'd like to have an option to skip installation of optional dependencies. Currently, this option sadly isn't supported even at the official Heroku node.js buildpack.

I believe the user should have the optional to add some boolean Heroku environment variable specifying weather to install optional deps. or not.

All the three package manager supports the option --no-optional. This saves time of installing testing or lintinting packages unnecessary for the final deployment.