Vanity address generation

Description

Oniongroove could have a built-in mechanism for generating a Vanity Onion Service address.

Tasks

• Vendorize Onionmine.
• Implement the needed configuration options so Operators can specify that the .onion address for the service should match a given pattern.
• Update the configuration step to run Onionmine if needed, in a way that only a single key is generated.