Apply alphas to individual nodes

This is my first pull contribution to a proper codebase, so any guidance would be great.

I've addressed issue #1199 regarding applying alphas to nodes in a similar manner to node colouring.

I haven't added any more tests, but have tested with some of the drawing examples, to make sure that they have the expected behaviour.

I haven't yet modified edges, since there is a lot more colour handling code in the `draw_networkx_edges` function.