Allow banner to be a function so banner can change depending on app state, or environment state

Currently, the banner is assigned on boot:

``` js
var banner =
"######################################################################" +
"# Welcome to joescrabshack.com #" +
"# #" +
"# All connections are monitored and recorded #" +
"# Disconnect IMMEDIATELY if you are not an authorized user #" +
"######################################################################";
vantage
.delimiter('appsvr:3000~$')
.banner(banner)
.listen(3000);
```

However this means that the context can only be when the banner was registered.

Instead, if the api could accept a banner, ie

``` js
vantage
.delimiter('appsvr:3000~$')
.banner(() => {
if (process.stdout.columns < 70){
return;
}
return banner;
})
.listen(3000);
```

Then the banner could be changed depending on the app state, or environment state. Some benefiting examples
- process.stdout.columns could be inspected to hide/resize wide banners (this currently only works if the same size shell as what booted the app in the first place is used)
- status of the running application could be returned, maybe a list of the last X exceptions
- Notifications that migrations need to be run
etc