[Markdown] Add support for YAML Header

[GProulx]

Hi @stkb !

Thanks for that great extension!

I currently use Markdown files with DocFx to generate documentation and YAML Header helps me keep extra information on markdown files.

Without this fix, rewrap generate something similar to that:

    ---                              ¦    ->      ---  ¦
    uid:                             ¦            uid: title: "High Level Architecture" ¦
    title: "High Level Architecture" ¦            ---  ¦            
    ---                              ¦            

So I created that PR to allow me to fully use your extension on whole documents.

Tests execution output:

(rewrap) C:\Code\GitHub\Rewrap>do test

Running Core tests
Passed: 249; Failed: 0; Errored: 0


(rewrap) C:\Code\GitHub\Rewrap>

Thanks.

[stkb]

Hi @GProulx! Thanks for this, though I decided it should be done, in a more "proper" way, i.e. making sure it the header only applies if it's at the top of the file. I've added it to some changes I was already working on.

It isn't yet in the current version, but you can try it if you like by downloading & installing v1.16.0-beta.1.