Add a COLOR flag for optional dependencies

[anko]

At the moment, we've got these colour-related flags (quoting man pacgraph):

     -t COLOR --top=COLOR
         Color of packages which are not dependencies.

     -d COLOR --dep=COLOR
         Color of packages which are dependencies.

It would help to have a separate colour for optional dependencies.

I like to use pacgraph to understand the relative sizes of packages that I might uninstall to make space, and currently optional dependencies are shown with the colour of the "not dependencies" (-t), which isn't wrong, but could be better.