Add a "clear" button to the console

I made a few changes but I kept each one in a separate commit to make it easier to see what each one does.

1. code cleanup, change hidebutton to togglebutton  https://github.com/tekay/jsbin/commit/54224283f5ca6e4cdeaa9e7e9dd5a8537bdb2d23
1. add a "clear" button to the console (currently the only way to clear it via the UI is to close and repopen the console) https://github.com/tekay/jsbin/commit/a43b51df84fec4451e1d9143a0ebd16c2c3e2c12
1. fixed the keyboard shortcut to clear the console in chrome on a mac (cmd+l just sets focus to the address bar) https://github.com/tekay/jsbin/commit/a4593fe3990d3bb60b5192d86f86f391e36d8dd8
1. update the console "clear" buttons title to "cmd+k" if on a mac