Support Markdown for drafting email

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**
There are so many wonderful and native Markdown editors in AppCenter. I don't know if it's possible, but my idea would be that I can simply paste a Markdown text that i formulated there into the Mail compositor and that this formatting will be sent that way.
This would help me a lot to write clear and structured emails.

**Describe the solution you'd like**
When I write a new mail, it should be possible to insert and display existing Markdown text in the compositor as well as format new text there in Markdown.
Of course, it would also be nice if emails from other people that have been formatted in Markdown could be displayed to me as such in Mail.