Add diw and daw to vi-mode

[fukamachi]

Add operator-pending state and text-objects to vi-mode, and add diw and daw as the first step.

diw and daw are commands to delete the word under the cursor. Also works with other operators, like ciw or yaw.

It does not work in visual state yet to expand the visual region. It will be added in the later changes.

[cxxxr]

Thank you!