Allow templating in conditions

Mergify conditions can now use the template syntax before they are evaluated.

For example, you could write a rule that uses the pull request author login in a matching condition:

pull_request_rules:
   - name: post a comment if the PR title contains the author login
     conditions:
       # True if the PR title contains the author login
       - title~={{ author }}
     actions:
       comment:
         message: You seem egocentric!