External importers

[costypetrisor]

• added a setuptools entry point hook for importers called "passpie_importers"
• added another exception for the default importer's exception handling

[landscape-bot]

Code quality remained the same when pulling b20e0f9 on costypetrisor:external_importers into 6e84de0 on marcwebbie:master.

[marcwebbie]

@costypetrisor thanks for the great work. Let's add some info to the README now.. :smiley: