Feature: Install dependencies for `init.js`

[ryan-roemer]

Currently, init.js can only require locally available files in the archetype and standard Node libraries. It cannot require normal npm dependencies because an archetype's dependencies aren't installed. (See #32 ). This is actually a good thing because we don't want to have the time of a full npm install before starting the template prompts because in our common case, the prompts don't need nearly any of those dependencies.

Task:

• Create a scheme for having dependencies from npm available to init.js.
  • Strawman: Add a dependencies field to init.js. If present, manually npm install DEP1 DEP2 etc. in the extracted/ directory so init.js can use them.

/cc @coopy