Allow html diffs to be interrupted / cancelled about daisydiff HOT 3 OPEN

Perhaps it would be better to find out if there are any memory leaks and fix 
them first?

I am not sure if DaisyDiff was designed in order to run with gigantic files. 
See also issue 21 and issue 23

There's no memory leaked beyond the end of the operation.   It's just that as 
daisydiff/rangedifferencer go through their machinations they slowly create a 
giant set of result definitions.  This may be the result of a bug that occurs 
for my specific data, but I don't really think so. Doubtless there are 
improvements to be made to daisy diff's memory usage.  But it may never be 
achievable to make it handle absolutely any data in limited time and memory.

However, as long as a running diff can be cancelled externally, mitigation for 
impossible diffs is feasible.

I don't consider this issue to be a defect report but an enhancement request 
(but I can't set that).

Think of it as a solution to 
http://code.google.com/p/daisydiff/issues/detail?id=21#c5 that is based on the 
observation that daisydiff takes considerable time to consume memory.