Add some more example grammars and a way to put them into the online editor

[birkenfeld]

The blank editor is hard to get started with. It would be useful to show/list several example grammars, and be able to put them into the editor to have something to start playing around.

[nathanielknight]

I think this is a great idea, and would be happy to work on some of the front-end plumbing to get it done.

Some example grammars that occur to me are:

• numbers
• URLs or Emails
• S-expressions
• JSON