Support for vanity addresses

Description

Onionspray now supports vanity address generation. It would be useful to have this exposed through the Ansible role.

Configuration could be like this:

onionspray_projects:
  - name: "example"
    hardmaps:
      - upstream_address: example.net
        vanity_filter: exa

This would generate a .onion address beginning with exa.

Tasks

• Add support for vanity addresses, directly in the onionspray_projects variable, for both softmaps and hardmaps.
• Test the feature through CI.
• Document the feature.