Added extension for wesQ3/vim-windowswap

Hi, I've added an extension to integrate with [my WindowSwap plugin](https://github.com/wesQ3/vim-windowswap).

The extension shows an airline indicator of which window has been marked for swapping. I followed as an example one of the other extensions, but please let me know if I made any mistakes or if you'd like me to change something.

Thanks for your work maintainting airline; I really enjoy it.