Added way to quickly calculate time/text changes between subtitle sets
gh-814, fix for multi-line italics, with tests
add linebreak mapping for ssa, plus tests, gh-692
Revert "add linebreak mapping for ssa, plus tests, gh-692"
ssa it too screwy to think about and just adding the /N to the
mapping doesn't help matters when you download.

This reverts commit b5f3be505fa87992f35be93ebad2a3c38d55efda.
unnecessary import
more test cases
changed s.tag to use element_tab(s)
add embedded span tags to the test data, for future use
test
Better way to handle nested spans/brs.

I think this solution is actually correct.  It makes a previous test
work.