Add message formatting options

[CubicrootXYZ]

Add options on how text should be formatted in Matrx, e.g. HTML, plain text, ...

I already mentioned this in #21, but wanted to open this separate issue to let you know that this is a really important feature for me and to letting you know, that I am planing on implementing that myself and then opening a PR for that. So we do not work on the same feature.

[eikendev]

Hi,

PRs are very welcome!

If you give this a go, please first look how Gotify offers a way to specify how the text should be displayed. This will help others if they try to migrate from Gotify.

[CubicrootXYZ]

Thanks for the reminder, would have forgoten to check that out else. I will try to implement the client::display from here: https://gotify.net/docs/msgextras