Add command to clear feedback from the exchange

[jhamrick]

Currently, you can use nbgrader list to remove released assignments:

• nbgrader list --remove removes released assignments
• nbgrader list --remove --inbound removes submitted assignments
• nbgrader list --remove --cached removes cached submitted assignments

But there is no way to remove feedback that has been released. There should be a corresponding behavior to nbgrader list such as nbgrader list --remove --feedback which would remove returned feedback.

[enisnazif]

Hey - i'm happy to work on this :)