Show collection id in prompt

[jisantuc]

Overview

This PR:

• shows the collection id in the prompt when in the collection context
• removes the collection id from the prompt when un-setting the collection
• adds a few convenience methods for specific prompt situations

Checklist

• Description of PR is in an appropriate section of the changelog (please use chan)

Demo

stac > set root url https://franklin.nasa-hsi.azavea.com
stac https://franklin.nasa-hsi.azavea.com > set collection aviris-ng
Set context to collection aviris-ng
stac https://franklin.nasa-hsi.azavea.com aviris-ng > unset collection
Returning to root context
stac https://franklin.nasa-hsi.azavea.com > 

Testing Instructions

• run the demo