Increase JSON nesting to allow for heavily nested patterns

Nesting limit changed from default of 20 to 1000, which is more than
enough to work with the examples in #3 #4 (except the giant email
regex which fails because it's invalid JS regex syntax)

Also display helpful error to insane users who manage to breach the
limit (although the parser hits the Ruby recursion limit eventually)
Monospace font for input box

Closes #9
Merge pull request #20 from dbr/nestedness

Increase JSON nesting to allow for heavily nested patterns