Fix text objects when :set keymodel+=stopsel.

Thanks for fixing #25 and #26 so quickly. However, the text objects still acted funny with my personal Vim configuration. Turns out that's a side effect of my changed `'keymodel'` option.

With that setting (which is set by :behave mswin), movement of cursor keys stops the visual selection, but the text objects are based on that. Replace the use of `<Left>` / `<Right>` with the canonical `h` / `l`; those are not affected by this setting.