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'
    nmap <expr> <Tab> sneak#is_sneaking() ? '<Plug>SneakNext' : '<Tab>'

With above mappings, type `gssssss...` to jump and leave default `s` behavior if sneak isn't active.