Improve yeoman interactive UI to list sub-generators

[lbeschastny]

Feature request

While yo --generators shows all available generators including sub-generators, yeoman interactive UI only work with main generators, completely ignoring sub-generators.

Interactive UI purpose is to simplify working with yeoman and installed generators. But many generators have sub-generators, which are not accessible from interactive UI.

I could think of two possible ways to include sub-generators:

1. Sub-generators could be listed together with main generators.
2. Alternatively, interactive UI could show a second prompt to let users chose a particular sub-generator to run in case there is more then one option available.

Similar proposal was made in #397, but the issue was closed by OP for no apparent reason,

[sindresorhus]

I would go with 2. Showing them in the main list would make the list very long and noisy.