Add params to at-command & helper, expose take_snapshot

[smartin015]

Continuing work on #16.

• Update @bedready and check_bed to support additional parameters for selecting saved references and custom match percentages.
• Add case-insensitive matching (e.g. @BEDREADY, @BedReady, and @bedready all perform the check).
• Expose take_snapshot method as a helper so snapshots could be acquired automatically (e.g. upon print completion)
• Deduplicate image comparison code
• Show both the source and test image in popups

To match against filename.jpg with a match percentage of 80% via at-command:

@BEDREADY filename.jpg 0.8

Plugin example:

def on_after_startup(self):
  helpers = self._plugin_manager.get_helpers("Bed Ready", "check_bed")
  if helpers and "check_bed" in helpers:
    self.check_bed = helpers["check_bed"]
    self.take_snapshot = helpers["take_snapshot"]

# Later in the plugin....

self.take_snapshot("new_snapshot.jpg")
self.check_bed("new_snapshot.jpg", 0.8)

Example of popup changes:

[jneilliii]

I think maybe your example for using the helper is wrong. Should be something like this?

def on_after_startup(self):
  helpers = self._plugin_manager.get_helpers("bedready", "check_bed", "take_snapshot")
  if helpers and "check_bed" in helpers:
    self.check_bed = helpers["check_bed"]
  if helpers and "take_snapshot" in helpers:
    self.take_snapshot = helpers["take_snapshot"]