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`.

@haya14busa
But it matches too much, especially combined with the two-key highlighting feature. Perhaps we should provide a config option to let the user decide if they want the consistence or the two-key highlighting