drupalmoduleupgrader 8.x-0.6

- Hopefully finalized the architecture of DMU now, with the code cleanly separated into Analyzers (add things to the report), Converters (actually change the code), Cleaners (clear out old D7 cruft post-conversion), and Indexers (which find the things to analyze/convert). See https://www.drupal.org/documentation/modules/drupalmoduleupgrader/contri... for docs.
- Analyzer issues now have Markdown support so you can easily input `code` and the like.
- Added a --backup option to the dmu-upgrade command.
- Conversions of various drupal_*_form() and form_*() methods covered in this change record: https://www.drupal.org/node/2121003
- Conversions of cache_get()/cache_set()
- The beginnings of Entity/Field API conversion; more to come!