core/source: use import error
uses the more broad ImportError
instead of ModuleNotFoundError
reasoning being if some submodule
(the one I'm configuring currently is
my.twitter.twint) doesn't have additional
imports from another parser/DAL, but it
still has a config block, the user would
have to create a stub-config block in their
config to use the all.py file
* initial pushshift/rexport merge implementation, using id for merging
* smarter module deprecation warning using regex
* add `RedditBase` from promnesia
* `import_source` helper for gracefully handing mixin data sources