Improve output of show

[Theodus]

The show command output should be enhanced with the following:

1. Mark which of the installed packages are being used
2. List package versions available on Cloudsmith that have not been installed

[SeanTAllen]

@Theodus do we want to do this before the 0.2.0 release? I think it sounds like a good idea.

[Theodus]

Partially implemented in #45. I'm thinking that show should have an option to display the latest package on each channel. So only the most recent nightly and release versions for each package.