Add Examples to CLI

[mattdesl]

I've got some examples so far, but most of them are more 'unit tests' to make sure it all works.

It would be great if the CLI could run a "Example Viewer" mode before generating a new sketch. Such as:

canvas-sketch --new --examples

The browser will list a thumbnail grid with examples, and clicking one will generate it from the template and start the server on that file.