Preliminary copy paste support for mime

 Support for copy/paste of images, files and html.
 Still needs work. Pasting images is a no-ip.
 Pasting html just adds the html in-line in the md
 Pasting image files works, but doesn't copies the image in the notebook
 same for any other file, just links to it. Inserts dummy link for
 remote files.
Simplify insertFromMimeData. Only images and links is treated specifically.
Automatically copy images to notebook folder won't be implemented, as
mikidown manages plain text md file, not capable of managing image files
reference in notes. Close #21