Configurable deadline reminders

[xuanxu]

This PR adds an optional feature to the ropensci_reviewers responder to set a reminder when a reviewers is assigned.

Reminders are configured in the ropensci_reviewers responder config with the reminder option. The parameters for the reminders are:

• days_before_deadline: How many days before the review's due date the reminder will be posted (default 4)
• template_file: The template file to be used for the reminder. Passed variables to ir are: reviewer, days_before_deadline & due_date

Example:

    ropensci_reviewers:
      only:
        - editors
      no_reviewer_text: "TBD"
      add_labels:
        - 3/reviewer(s)-assigned
      remove_labels:
        - 2/seeking-reviewer(s)
      reminder:
        days_before_deadline: 2
        template_file: reminder.md

Ref: #56

[maelle]

Thank you! This looks good. @mpadge any comment?

After merging I'd do a manual deploy.

[mpadge]

Looks great to me - thanks @xuanxu

[maelle]

Now deployed thanks to https://help.heroku.com/CKVOUPSY/how-to-switch-deployment-method-from-github-to-heroku-git-with-all-the-changes-app-code-available-in-a-github-repo