browser keypress selects page key

Javascript onkeypress handler which marks and clicks on a matching key
if it is pressed in the browser.

Increases Vim page size by 11kB (approx 10%) because each key requires
a class name for unambiguous character identification.  With gzip
compression the difference is just 1422 bytes (though still 10%),
so assume this is (or at least will be) a worthwhile trade-off.