Enable Markdown and HTML notifications. (#288)

This change enable defining the notification body as HTML or Markdown. This can be very
useful to have more user-friendly notifications such as:
* applying a heading style to the `{watch_title}` to make it stand out
* creating clickable links using the `{watch_url}`, `{preview_url}` and `{diff_url}`.

Changes
=======
* Add a `notification_format` to the notification settings, defaults to plain text.
* Use the `body_format` parameter of Apprise's `notify` method.

Co-authored-by: Malo Jaffré <malo.jaffre@dunnhumby.com>

changedetectionio/store.py

@@ -47,6 +47,7 @@ class ChangeDetectionStore:
                     # Custom notification content
                     'notification_title': None,
                     'notification_body': None,
+                    'notification_format': None,
                 }
             }
         }
@@ -73,6 +74,7 @@ class ChangeDetectionStore:
             'notification_urls': [], # List of URLs to add to the notification Queue (Usually AppRise)
             'notification_title': None,
             'notification_body': None,
+            'notification_format': None,
             'css_filter': "",
             'trigger_text': [],  # List of text or regex to wait for until a change is detected
             'fetch_backend': None,