Add option to not highlight results after <Plug>(easymotion-sn)

[hanschen]

After using <Plug>(easymotion-sn), easymotion will highlight all matches after pressing Return if hlsearch is set. Add an option to not highlight search matches (the highlights after pressing Return) even if hlsearch is set.

I'm not sure if some of the current options (g:EasyMotion_landing_highlight, g:EasyMotion_move_highlight) are supposed to control this, but I tried setting both to 0 and I still get highlights after running <Plug>(easymotion-sn).

If this is a bug please let me know, and I'll try to make a simple case that can be reproduced.

Related: #215