Update doc: Remove VSneakNext & VSneakPrevious

Ref: #91
Merge pull request #92 from haya14busa/patch-2

Update doc: Remove VSneakNext & VSneakPrevious
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()
update readme
fix: <Plug>(SneakStreak) should be quasi-exclusive
<Plug>(SneakStreak*) xmap, omap

- provide <Plug>(SneakStreak*) in visual-mode and operator-pending mode
- stub doc for sneak# API
- remove doc for deprecated commands
visual-mode: eliminate flicker

- buttery smooth (?)
- avoid need for <esc> with xnoremap
- remove spurious 'range' markers
- sneak#rpt(): use v:count1 instead of requiring parameter
fix #96: 'Cursor' hl group may not exist
don't highlight same buffer in other windows

- improves performance
- reduces distraction
Disable "clever-s" by default

To enable "clever-s", add this to your .vimrc:
    let g:sneak#s_next = 1
fix #98: streak-mode: clear highlight after target
fix #27: multibyte highlighting!

fix a (literal) edge case: matches near edge of screen were not
considered on-screen.
fix #101: 'Cursor' highlight missing ctermbg
Add an option to support optional regex input

Fix #104. This should support more flexibility in customisation. With
this option enabled, only input text not containing "\v, \V, \m, \M" are
treated literal.