Add support for formatting in messages

[rihardsk]

Closes #9, closes #26

This basically is how the hangups client itself handles formatting. It supports _italics_, **bold**, <del>strikethrough</del>, http://example.com/urls and maybe something more.

[tulir]

This is not the correct way to bridge formatting, but I guess it's not worth making a proper parser if Google will finally replace Hangouts with Google Chat next year

[rihardsk]

Out of interest, what is the proper way? Should I have parsed formatted_body instead?

[tulir]

Yes. body is plaintext, not markdown. Parsing the HTML in formatted_body is the correct way to handle formatting. (for example: facebook bridge parser and telegram bridge parser which use this mautrix-python utility)