parseEmailToMarkdown function added for stream email posts

[Sloff]

This improves the look and feel of emails within the stream

[yurikuzn]

Hi,

I can't apply this commit. I'm not sure whether this will function reliably.

[Sloff]

Would it change your mind if I add several unit tests for the function?

[Sloff]

Or I could simply provide a couple of example emails and how they appear in the stream

[Sloff]

There is also a composer package that can accomplish this: https://packagist.org/packages/league/html-to-markdown

[yurikuzn]

I don't have time now. I will leave it aside for the future.