Introduce Sanitizer class and text module

This class can currently be used as a drop-in replacement for collapsing
ellipses and whitespace, but will be expanded shortly to cover all
cases.
Port sound strippers over to use Sanitizer class

This also splits up the old atts sound stripper method into three
methods that provide the kind of desired sound stripping.
Port filename stripping into Sanitizer

This is already used by the audio stripper and will now also be used by
the MP3 generator's "remove existing" option going forward.
Get RE_FILENAMES from text module

Ths main init module should no longer define this on its own.
Add HTML rule to Sanitizer class

This will be used for the three core rules.