don't merge yet

I just wanted to get this pushed up. we can leave the pr open until everyone things it's good to merge. 

There is a lot here, but the main thing I focused on was subgenerators. I [started documentation for them](https://github.com/jonschlinkert/generator-assemble/blob/master/docs/sub-generators.md), but not all of them have been created. Since @doowb already put the helper/plugin generators together, it might be good to use those somehow when a user runs `yo assemble:helper` etc. But I'm not sure how that works yet. 

Also, I probably broke the gruntfile! sorry @hariadi, I haven't had a chance to test or fix issues - I'm still working on this. 

I anyone else wants to add to this pr, with templates, tests, fixes, whatever, please do!!! Let's make this a first class generator! (I might even add themes!)