CLI command to add an alias for a specific contributor manually

Suggested by @cdolfi on slack

We currently keep track of contributors to repositories (via the `contributors` data table), but since contributors can have multiple email addresses (which can be associated with commit activity etc), we also have a table thats called something like `contributor_aliases` for storing these emails.

if a particular contributor is known to have a particular email alias, having a CLI command to add that alias may be helpful.

However, i think a lot of discussion may be needed surrounding checks and balances for this (i.e. what if you try to alias an email to a contributor that is already assigned to someone else?)