Added an entry in test4 of the testing suite to check that the 'sortname', 'sortyear', and 'sorttitle' fields are treated properly when sorting citations.
Added two new options to test4 in the testing suite, checking that options 'sort_case' and 'sort_with_prefix' are operating correctly.
Added 'citeextract' check to test3 within the testing suite. This required fixing a couple of bugs within bibulous.py's 'write_citeextract()' method, and also modified bibulous_test.py's 'check_file_match()' to allow multiple output files and multiple target files for comparisons.