Add user-defined dictionary feature for find motions

Example:
    let g:EasyMotion_userdict = {
    \ ,'\d' : '\d*\zs\d'
    \ ,'??' : '\v[!"#$%&''()=~|\-^\\@`[\]{};:+*<>,.?_/]+'
    \ , 'ee' : '[eæœèéêëēėęěə]'
    \ , 'ww' : '\(\<.\|^$\)'
    \ , 'jk' : '^\(\w\|\s*\zs\|$\)'
    \ }