OneJS control generator

Modified `yo onejs` to prompt the user to either generate an app or a control.

The control option:
* Creates a dir `src/{control_name}`
* 4 files in that dir:
     * {control_name}.html
     * {control_name}.less
     * {control_name}Base.ts
     * {control_name}Model.ts
* A Chai test stub: `test/{control_name}.test.ts`

The original `yo onejs` functionality is untouched if the user types in "app" rather than "control" at the prompt.