Added a quick snuffling help functionality

Hi!

As a follower of the 'docs or it doesn't exist'- idea I added a functionality which makes it easy for a snuffling's author to add a help button to his snuffling.
A 'Snuffling Help'-button will be added when 'help_button_from_doc(True) is called within snuffler. A click on that button opens a help window, containing the plain text given in the snuffling's __doc__ string.
What do you think about that idea?

Cheers and happy new year!

Marius