Move dependency functionality into the library

[dcrockwell]

The automatic dependency management might be needed for local testing.

Getting: Error: Couldn't find preset "es2015" relative to directory "/Users/dcrockwell/Code/stimpak-github" when doing gulp-test on a local generator that has a symlinked subgenerator

If the dependency management was made an official feature of stimpak, we could detect which dependency strategy to use internally, and we can unit test it much better than with the CLI.