Reduce spamming of octoprint.log

Thanks to @pto199 for logging #13 about an excessive amount of self._logger.info calls in the plugin - causing bloat to the octoprint.log . Even more thanks to @tideline3d who created #17 to address these issues (and easy for me to merge in).

Added @appearance variable to list of supported variables

This was a customer request so that @appearance.name could be used in the "device identifier" webhook parameter (reflecting the OctoPrint instance's name)

Added "verify ssl certificate" option and fix another issue with settings saving

This release adds the ability to turn off ssl certificate verification per hook (#10) and adds another fix for settings saving (the "Settings update detected" dialog that mentions conflicting settings) when clicking the "Send Test Webhook" button (#8)

Added optional cooldown for events

This version implements an optional "event cooldown" feature (#5) that allows you to specify how often the same event can fire. This can be useful for some events that fire quite often (eg "User Action Needed") and prevent it from spamming your webhook.

Custom Events!

Added the ability to subscribe to custom events - any events sent by OctoPrint (https://docs.octoprint.org/en/master/events/index.html#available-events) or any events sent by other plugins! #2

Fix settings saving!

This version has really only one fix (besides some code formatting changes): fixing the settings saving that has been plauging users for ~3 years! (see #1 and 2blane#19)

New maintainer!

This version does not change any core functionality, but updates internal values (author, update url, etc) to reflect the new maintainer. Please raise any issues at the new repository location: https://github.com/derekantrican/OctoPrint-Webhooks