Custom plugin directory scanning support added

[caronc]

Description:

Related issue (if applicable): refs #108

Support the loading of custom plugins.

# Start it up:
# /config is used for a persistent store for your configuration.
#         You do not have to mount this if you don't intend to use it.
# /plugin is used for a location you can add your own custom apprise plugins.
#         You do not have to mount this if you don't intend to use it.
docker run --name apprise \
   -p 8000:8000 \
   -v /var/lib/apprise/config:/config \
   -v /var/lib/apprise/plugin:/plugin \
   -d caronc/apprise:latest

You can set the APPRISE_PLUGIN_PATHS environment variable to over-ride the default path.

Note: This merge needs to hold off as while testing, i uncovered a bug with Apprise fixed here.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] Tests added