Ebookmaker

- fixed bug for pickled jobs; dc was previously passed silently via builtins in options
- fixed bug in cover generator when url is a file: url

- config files can now be used to provide defaults for most command line arguments.

- added updates so that gutenberg canonical url is always https
- updated libgutenberg requirement

- fixed parsing of rst files in MyDocUtils
- fixed undefined options in TxtWriter and PdfWriter

- updated libgutenberg requirement. itd DublinCore class depended on the installation of '_' that was removed in ebookmaker 0.4.1a1

- added strip_links command-line option. turn it off to stop EbookMaker from stripping links in EPUB and MOBI output
- the test_htm not longer tries conversion to pdf or rst
- fixed conversion failure when html contains '<pre><code /></pre>'
- corrected install directions
- readme is now markdown
- fix python 3.6.7 compatibility issue when dbm_gnu not present
- added tests for loading parsers, loaders and packagers