Add open vim buffer example

There's probably a better way to do this, but I at least wanted to start the conversation. Here's what I have so far. The code is pretty self-explanatory and leverages the existing SelectaCommand. It works well so far.

Until you have a vim plugin I can use, I've started collecting these in my own: https://github.com/michaelavila/selecta.vim

I don't know if anyone else will find this valuable, but I use buffers and selecta heavily. I'm happy to have selecta to make the experience better.
Shouldn't this be `:b` instead of `:e`? I think that `:e` will trigger file loading autocmds that `:b` won't, like detecting syntax.