Make close icon semi transparent, in case it overlaps the content

This mostly reverts "Remove redundant CSS rule preventing simple change of background color",
commit 2544925700b86531d4b0337541eb9a9bbfae9670.
- open/close configuration option
Use promise for Ajax loading
Remove 'manual' binding option, just use $(selector).featherlight
Merge config.autostart,context,selector to $.featherlight.autoBind
Change argument order of $.fn.featherlight for consistency
Make type a simpler config.
This also insures that a single type can be set at once.
Extract elementConfig
Attach trigger to set instead of each individual element
Fix fixture handling
New config: filter, for live selector. Use it by default
Allow config of style {ajax: 'url'}
Add otherClose configuration option
Check if event has been prevented before opening the dialog
Merge config with featherlight object
Update Changelog
Gallery: store  in featherlight object
Gallery: add before/afterImage callbacks and store $current
Gallery defaults now accessible