Add Route Target AFI/SAFI

[grahamjohnston]

This change adds a new AFI/SAFI of type "Route Target" to the list of AFI/SAFI Choices. This PR is related to issue 137.

• README was not updated as this change is so minor.
• This change does not require any changes to configuration settings
• I didn't update any test cases as existing test cases only covered most, but not all of the existing AFI/SAFI.

Change Notes

Added a new option to the list of choices in the AFISAFIChoices class.

Justification

This choice was missing from the list of available options, and I use it in production networks.