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/forms.py

@@ -6,6 +6,8 @@ from wtforms.fields import html5
 from changedetectionio import content_fetcher
 import re
 
+from changedetectionio.notification import default_notification_format, valid_notification_formats
+
 class StringListField(StringField):
     widget = widgets.TextArea()
 
@@ -203,6 +205,7 @@ class commonSettingsForm(Form):
     notification_urls = StringListField('Notification URL List', validators=[validators.Optional(), ValidateAppRiseServers()])
     notification_title = StringField('Notification Title', default='ChangeDetection.io Notification - {watch_url}', validators=[validators.Optional(), ValidateTokensList()])
     notification_body = TextAreaField('Notification Body', default='{watch_url} had a change.', validators=[validators.Optional(), ValidateTokensList()])
+    notification_format = SelectField('Notification Format', choices=valid_notification_formats.keys(), default=default_notification_format)
     trigger_check = BooleanField('Send test notification on save')
     fetch_backend = RadioField(u'Fetch Method', choices=content_fetcher.available_fetchers(), validators=[ValidateContentFetcherIsReady()])
     extract_title_as_title = BooleanField('Extract <title> from document and use as watch title', default=False)