Allow for variables in comment templates

It would be nice to have possibility to include some dynamic variables in comment, like:
- pull request name
- pull request id
- branch and base branch
- author
etc.

Start by tracing how comment templates are read from configuration and how the Probot app receives pull request and issue events. Identify the supported event data needed for pull request name, ID, branches, and author, then define a documented variable syntax and verify that configured templates render those values.

Written by the indexing model from the issue text.