Add <Plug>SneakNext & <Plug>SneakPrevious for xmap

Ref: #90

The users can configure more easily with this commit for <Plug>SneakNext & <Plug>SneakPrevious

```
map : <Plug>SneakNext
map : <Plug>SneakPrevious
```
Edit: I removed an unneeded blank line and push -f. Sorry.