Fix text objects when :set keymodel+=stopsel.

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.
Merge pull request #27 from inkarkat/fix-keymodel-stopsel

Fix text objects when :set keymodel+=stopsel.