ux: improve the `show` output

[gilligan]

This PR aims at improving the UX around the npins show command by making the output easier to digest and more condensed:

$ npins show
   nixpkgs: GitHub:nixpkgs/nixos@8f39ad3da14ce78daf249c8291a4e43a17bbcf98 (nixpkgs-unstable)
   pre-commit-hooks.nix: GitHub:pre-commit-hooks.nix/cachix@433808cba23975201a48a3bb8ebc76029191fafd (master)

Relevant changes:

• Refactoring the mkPin macro into explicit Pin types: this was necessary in order to provide custom Display implementations for different pin types
• Extracted Pin to src/pin.rs module
• Changed the show function to yield the above
• Added ansi_term crate and used it to print pin names in bold in the show command

[piegamesde]

Superseded by #23.

I tried to incorporate as many changes from here as possible, if I missed any please let me know or simply open a new pull request.