Add options to configure the 'source' remote.

[zanerock]

Overview

With the new ability to 'follow' a source repo, we need to be able to manage that. This will be added as configurable options reached through 'sync'.

Depends on #128

Deliverables

• projects sync --unfollow will remove the source remote
• projects sync --follow <url> will add a source remote
  • If there is a source remote, this results in an error.
  • If successful, suggests running liq projects sync --source to pull in changes.

Notes

We will consider supporting multiple sources in the future. For now, we keep it simple till we have a compelling use case for multiple sources.