[Suggestion] Move to .md over .htm

I find that I am growing to like ownnote more and more. So out of the gate a quick thank you for sharing your work.

I do wish, however, that instead of htm file types that the use of Markdown might be considered. This would allow direct editing of a file with any number of Markdown friendly editors (whether internal or external to owncloud).

Rendering of md files is works quite well in owncloud as does the editor.