Add the ability to use features with nphysics (just like ncollide).

This allows the user of nphysics to do (for example for 3d f32):

[dependencies.nphysics]
features = [ "3df32" ]
git = "https://github.com/sebcrozet/nphysics"

instead of:

[dependencies.nphysics3df32]
git = "https://github.com/sebcrozet/nphysics"