Refactor rendering of title via `config.title_format`

[SmileyChris]

Description

Make title_format work for markdown as well as restructured text. Fixes #609.

Checklist

• [x] Make sure changes are covered by existing or new tests.
• [x] For at least one Python version, make sure local test run is green.
• [x] Create a file in src/towncrier/newsfragments/. Describe your change and include important information. Your change will be included in the public release notes.
• [x] Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• [x] Ensure docs/tutorial.rst is still up-to-date.
• [x] If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

Thanks for the PR

For this PR we can continue with the hardcoded support for RST and MD.

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I think that in an ideal case, the builder should not care about makrdown or restructured format.

In the future we can also have AsciiDoc, or other formats.

It would be nice for title_format to export all the required variable to be able to build a multi-line jinja template.