Rudimentary way to specify other required bundles

[shawncplus]

Add a check-bundles command in util/ that will go through a specified (or each) bundle's package.json looking for a ranvierDeps property and verify that those bundles have been installed.

the ranvierDeps config should look like this:

"ranvierDeps": {
  "bundle-example-lib": "https://github.com/some/repo[#branch=master]"
}

The install-bundle should be updated to allow for the #branch part of a url.

I don't want to go into a whole situation of package locking or any of that insanity. Just warn if they're not installed and suggest installing with install-bundle