Adds support for React Native templates.

[skellock]

Closes #730.

ignite new SweetBabyJesus \
  --empty \
  --react-native-version 0.42.0-rc.3 \
  --react-native-template navigation

This was quick & dirty, but it'd be nice to integrate at a deeper level.

For example, I'd like to explore embracing their templates as being the blessed way to do our app templates. It's kinda awkward having both.

Their fallback of checking react-native-template-* on npm is 💯 (see facebook/react-native#12548).

[jamonholmgren]

Nice, and I agree, let's work on using their templates if we can.