Indicate when a dependency is only a dev dependency

[hobofan]

We should display some indication for dev-dependencies, and probably also sort them to the bottom of the list of all dependencies for a module.

[alexeagle]

Another option would be not to show them at all. For a user of the module, they are not relevant. The only ppl who might care are those who maintain the module, or perhaps when we have #51 it will be useful to see that X dev-depends on this module so we don't undercount.