Renamed some tests, introducing subsidiary directories
for fb2, docx, epub.
Cleaned up tests in cabal file.
Combined dokuwiki-writer and dokuwiki_inline_formatting tests.
Rewrote features test to remove all unimplemented features.
There are now all three examples of where an image can be included in
the test.
1. Cover image
2. As a spine elemnt
3. In the document
Tests have also been added to make sure that the mediabag contains all
these images after processing.