Implement %AutoFormat.RemoveEmpty, end to start ref, and injector rewind.

Implement %AutoFormat.RemoveEmpty, end to start ref, and injector rewind.

Injector rewind: Injectors can now use the method rewind() in order to move
the input index backwards, so that they can reprocess tokens (other injectors
are not affected by a rewind). This functionality was necessary to implement
nested node removals in %AutoFormat.RemoveEmpty.

End to start ref: To facilitate rewinding, HTMLPurifier_Token_End now
maintains a reference called $start to the starting token for their node.

%AutoFormat.RemoveEmpty removes empty nodes. Lots of people have requested
it, so here is a partially effective implementation. Because it is implemented
as an Injector, it's not possible for it to handle newly introduced empty
nodes by later validators, specifically auto-closing and child validation.
The Injector is only meant to be used on HTML-ish languages.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>