Add aliases to gh-pairing-with

[abigailychen]

Be able to add an alias or nickname to an individual or in a group. Examples:

gh pairing-with "my-pal" monalisa

gh pairing-with schustafa stephanieg0 //adds multiple users to alias

gh pairing-with buddies
gh pairing-with --alias buddies
gh pairing-with --alias buddies schustafa stephanieg0 //adds multiple users to an alias group

gh pairing-with --alias kiran krhkt //adds a single user to an alias

gh pairing-with --list-aliases

gh pairing-with --delete-alias buddies

Sub-issues:

• Redefine an alias
• Remove an alias

Stretch goals:

• Add an individual alias
• Error message for if you try to add an alias that is already taken