Output on all html pages hidden by default.
Each div.output element gets an id.
Each div.output element gets an id; added show_hide_output.py.
Show/ hide buttons added to each output block.
js file added to header of all pages, works for show/ hide output 0.
Show/ hide works for all individual output cells.
Show/ hide all buttons at top of page; but all pages except index have it.
Show/ hide buttons are only added to pages with output.
Merge branch 'show_hide_output'