Allow to set custom key modifier to key bindings

The `jk` keys is finger-friendly for Vim users, but able to set own key modifiers for them is a nice feature. For example, 

```vim
let g:move_key_modifier = 'C'
```

create following key bindings:

    <C-k>   Move current line/selections up
    <C-j>   Move current line/selections down

Of course, the user can use default plugin API to map any keys to any action.