Add 'add' command along with the 'foodchain' recipe

[Nitemaeric]

The new drenv add <recipe> command configures curated DragonRuby dependencies.

The first recipe is pvande/foodchain. foodchain is a package/dependency manager for DragonRuby that allows users to keep track of dependencies and be aware of new updates to libraries. It achieves the same goals as RFC - Package Management #6.

• Fully Decentralized
  • Pull what you want from wherever it lives.
• Git-Aware
  • Pin your dependencies to a particular branch, tag, or commit.
• Documentation
  • Borrow code with the confidence that you'll remember it came from.
• No External Dependencies
  • Everything runs inside the DragonRuby runtime you already have.

- pvande/foodchain's README

This command allows users to bootstrap installation.

New commands

• drenv add <recipe>

  Install/setup a pre-configured library or DragonRuby snippet.

  e.g.

  drenv add foodchain