make `EasyMotion#WB` and `EasyMotion#E` be consistent with vim's `iskeyword`

Make function `EasyMotion#WB` and `EasyMotion#E` be consistent with vim's `iskeyword` option.

@haya14busa
It matches a little bit much, especially combined with the two-key highlighting feature. Perhaps we can solve this problem by providing a config option to let the users decide what they want, the consistence or the two-key highlighting, or both.