If we are lacking an optional dependency, skip the tests that rely on it.

Signed-off-by: Marcin Bachry <hegel666@gmail.com>

The old test expected an incorrect wrapping due to the preview function
not using unicode properly, so it got the wrong length.

Signed-off-by: Johannes Löthberg <johannes@kyriasis.com>

Signed-off-by: Johannes Löthberg <johannes@kyriasis.com>

This includes:

- Splitting out methods of a class into stand alone functions, to make
  them easier to test.
- Adding unit tests to split out functions, testing HTML -> preview.
- Handle the fact that elements in lxml may have tail text.