SlackReporter with preformatted rich text

[vimagick]

Hi, I've updated my code ( #776 )

Three things:

• [x] Have you checked whether or not Mattermost supports Slack's rich text feature?

   Mattermost still use the original `{"text": text}`
  

• [x] How about users who want to opt out of this new behavior and have the old (non-rich-text) behavior? Should there be an option? What should be the default?

  Enabled only when `rich_text: true`
  

• [x] Please update CHANGELOG.md with information about this change

  Yes
  

[thp]

@vimagick Did you have time to look into these?

• [ ] Can you add 'rich_text': False, to lib/urlwatch/storage.py in the 'slack': section, so that it's easily discoverable?
• [ ] Can you add documentation for this to the Slack section in docs/source/reporters.rst

[thp]

@vimagick Needs rebasing, and there's still the two items from above. Thanks!

[thp]

Closing after no activity. Feel free to reopen and update the PR.