Browser piggieback connection convenience

[jgdavey]

This is mostly a sketch. So don't accept this.

Here's the idea:

:Piggieback browser

would use the common settings of http://localhost:9000 for the listener.

:Piggieback browser:9001

would use http://localhost:9001, etc.

I don't love piggiebacking on the :Piggieback command, but wanted to start the conversation.

[tpope]

I had been entertaining the idea of accepting a bare integer as a port actually, as that would be a nonsense value to pass in (whereas browser could be conceivably intended as a var).

[jgdavey]

That makes sense. I've updated the commit.