add `--quest` flag to `mod` command

[charlesHetterich]

Pre-requisite:

• Complete mod command #3
• Todor's design of the quest mechanism is finalized.

The current expectation (subject to change from Todor) is the following experience:

dot mod <domain> --quest <id>

• the mod command will clone (fork if gh logged in) a playground app. Adding the quest flag should do the same thing, but point to a specific branch of the repo
• We need to track some sort of metadata, so that when the user later does deploy, any important reference to the quest is available
• deploy command should be updated to be quest aware. Implementation depends on Todor's design

[charlesHetterich]

cc: @todortodorovic