Update plugin to use is.vim rather than incsearch

[nerd190]

first thank you! the readme still points us to incsearch.vim so that we can integrate it with easymotion, this has been taken over by is.vim, could we integrate easymotion into is.vim? you could then update the readme to use the updated is.vim instead, some of the original incsearch.vim was merged into vim 8, so using is.vim instead would be greatly apprectiated! thank you again!