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I am considering making an optional argument to the
constructor for specifying tab indentation.  If nothing was
passed it defaults to not handling tabs (better
performance).  If a number is passed, the string sequences
(lists of lines) would be wrapped with a generator function
to convert the tabs in each line with the expandtabs string
method.  The other option is to expand tabs all the time. 
If one is going to handle tabs it must be done on the input
because once it is processed (markup added) the algorithm
for expanding tabs becomes really compilicated.  Any
opinions regarding these options are welcome.

I think I should change the default prefix (_default_prefix)
to be a class member rather than initialize it with the
constructor.  (The default prefix is used to generate unique
anchor names so there are no conflicts between multiple
tables on the same HTML page).  I'm leaning this way because
a user may create separate instances of HtmlDiff (to have
different ndiff or tab settings) but   place the tables on
the same page.  If I left it the hyperlinks in the second
table would jump to the first table.