Browser piggieback connection convenience

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.