find_paragraph_text_object(): modified to be more faithful to Vim

These changes address to the following issues: https://github.com/guillermooo/Vintageous/issues/675.

A few notes:

-- When I cloned, the test suite had 7 failing tests (wonder if I've set up my dev environment incorrectly?). After these changes, the same 7 failures are occurring.

-- I am willing to help by adding some more tests related to paragraph text objects, but before doing so I wanted to get a sanity check on the code I've got so far. Suggestions welcome.

-- My suspicion is that find_inner_paragraph() could be simpler if I knew my way around the sublime module better. Does sublime provide an easier way to identify regions of lines having the same whitespace status? CLASS_EMPTY_LINE doesn't appear to help, because it's too strict.

I think you need to subclass the tests from BufferTest instead (but I'd need to doublecheck). BufferTest implements .set_text() and friends as methods. I'll confirm later.