Allow handler specific parameters in <gallery> (page number, etc)

For multipage media, people really want to be able to specify
if the image gallery should display page 1 or page 10. This
also allows other handler specific parameters like thumbtime
for videos, "lossy" for tiff files, etc.

Note, this only allows the handler specific options
(typically things that would change an image). Other options in
the thumb syntax like class, border, upright, left, etc are
still not supported (and mostly probably should not be)

Bug: 8480

Change-Id: Ib831d89ed8676deb2f44238ff9a23ce58ad4d2df