Allow object nesting for class generation

[TheDutchCoder]

Some parts of Tailwind already use this (like colors), but it would be nice if things like box shadows would also support object nesting.

The following code does not work:

    boxShadow: {
      foo: {
        bar: '0 10px 10px 2px red',
      },

It would be great if this would generate to shadow-foo-bar, just like how text-purple-500 does it!