Custom yank keys

This a potential fix for the issue I describe in goldfeld/vim-seek#6. It allows users to do

```vim
let g:yankstack_yank_keys = ['c', 'C', 'd', 'D', 'x', 'X', 'y', 'Y']
```

in their `.vimrc`, recovering the `s` and `S` keys for use by `vim-seek`.

I recognise that this would need some tests before a merge, and I'd also warn you that I really don't have a clue what I'm doing when it comes to vimscript, so treat this as more of a "request for comments" than a pull request.