Handle optional dependencies properly

[mrcnski]

Optional dependencies should not be treated as a "dependency" kind, of which there are only three -- regular, build, and dev. "Optional deps" should instead be handled as a boolean flag, which is what cargo does. A dependency can be both a build dependency and optional simultaneously, but currently cargo-deps will erase one of these.

Optional dependencies can be displayed with a dashed line.

[jplatte]

I'm working on this :)

[mrcnski]

Awesome! Let me know if you get stuck.