Also added rough documentation for insert mode.
TODO: Abbreviations should also expand on <CR> and <Tab>, but right now
adding <CR> to the mappings.add() has no effect, and adding <Tab> to the
mappings.add() breaks tab completion.
TODO: Rather than listing non-keyword characters in two places (ui.js
and mappings.js), they should be specified in one location (that could
someday be trumped by an 'iskeyword' option).