Add API: sneak#is_sneaking()

This api provides more flexible mapping using `<expr>`
Especially for customizing clever_s feature

Example:

    nmap gs <Plug>Sneak_s
    nmap <expr> s sneak#is_sneaking() ? '<Plug>SneakNext' : 's'
Merge pull request #93 from haya14busa/is_sneaking-api

Add API: sneak#is_sneaking()