Is your feature request related to a problem? Please describe.
The most common issues users report by using ThirdStats are problems when processing stats (like wrong numbers or never ending processing). And in most cases, there is no helpful output in the Thunderbird console.

Describe the solution you'd like

• Add a setting "Debug mode" to the ThirdStats options, which is disabled by default.
• When enabled, ThirdStats logs verbose messages to the console while processing stats
• The log could contain:
  • Processed Folder with message counts
  • Total counts

Additional context

See #413 and #445 for example.