Allow the user to specify that navbar items should be aligned right

[Munksgaard]

This is a potential fix for #48. I don't know if that's exactly what everyone is asking for, but it's certainly what I'd like. However, I have no idea if it somehow breaks the way that customItems work or something.

The idea is to add withItemsRight, which allows the user to specify that navbar items should be right aligned instead of left aligned.

[Munksgaard]

Another option that would provide a bit more flexibility would be to amend the Bootstrap.Navbar.items function to also take a list of attributes. That way you could pass in Bootstrap.Utilities.Spacing.mrAuto or Bootstrap.Utilities.Spacing.mlAuto at your leisure.