added HIDE_ARTICLE_DATE setting in article_info.html

Hi,
thanks for the great template!
I found myself wanting to hide the dates because 'pages' cant be displayed in categories the way i want to.
so I added this option. 
The naming is inconstant with the other settings (hide instead of show) because i don't know where pelican stores defaults and i didn't want to change standard behavior..

best, Alec