HtmlBlockProcessor preserves empty lines

Partial fix for #183. This has the same effect on empty lines in code blocks
as not using the html processor at all (which was eating some of the missing
newlines as reported in issue #183).

By doing `rsplit('\n\n')` the third newline (in each set of three) always ends
up at the end of a block, rather than the begining - which it less of an issue
for the html processor.

Also updated tests to indicate final intended output, although they do not fully
pass yet.