Add option to defer loading pixel

The facebook pixel tracking code is already being loaded async.

I propose changing this issue to:
"Add option to defer loading pixel"

And adding an option "Defer loading pixel until after page load" which when checked adds defer=true to the tracking code. This will tell the browser to wait to load the main tracking script. The stub will still be inserted on the page but that shouldn't have the same performance impact as loading the pixel library does.

@rhovland agreed!