concatenate a directory of files into a src file

@sethladd 

This converts the helloworld generator into one where the source is hosted in a separate directory (`examples/helloworld`), and a tool is used to generate a resource only source file (`lib/generators/helloworld_data.dart`). This lets us:

- have arbitrarily complex generator samples
- edit the code as if they were regular projects
- load the generator data in either a dart:io contex or a dart:html one

Thoughts?