Start parsing text objects and implement 'dw' command.

CC @kisielk 

Looking for initial feedback on the general direction.

Follow the same model as 'i'/'a', where we use onExit() to perform intended
action (in this case, transforming text object to cursor range and running
specified modifier function)

Discussion points

* Is this the right place for text object / cursor range conversion. Will this work for all cases? There's a question of how to implement stuff like '.' command, where we definitely need to remember last text object rather than pair of cursors. However, I think text object mode can set something like 'editor.lastTextObject' inside onExit() 
* Should the incremental key combo parser be factored out and generalized? Could probably be used in other spots as well.
* view.onVcommand has some pre/post logic such as finalizing history and performing repetitions. Either we do it manually every time or onVcommand persists in some shape or form (but definitely under different name)