Various minor additions to flash motions

This is what I have been using for the past 2 weeks.

I think some of these changes are desirable for your branch.  I'm just creating this pull request so you can review and keep what you want.  I actually made a few branches so you could merge in some but not all.  Or you can just copy-paste of course!

- You might not care about supporting SexyScroller.
- But if you drop those calls, please at least keep the little `redraw`; that makes the plugin feel responsive for users with a slow machine.
- "Do not flash if there are pending keys." - this is another responsiveness enhancement.  I wanted to do it earlier (before matches are calculated) but I was too scared I would break something!
- My `n` and `N` behaviour (following last search direction) is now more like Vim's default, but may be a break from how EasyMotion has worked in the past.