At the moment, every page in a PDF will be processed one by one using
tesseract. Since the processing of a single page is independent from every
other page, one can make use of multi-core machines.
This PR introduces a multiprocessing pool to process multiple pages
simultaneously. The amount of threads to use can be specified in the
environment variable `PAPERLESS_OCR_THREADS`. This will default to the
number of cores/hyperthreads Python detects for your system.
Rename exporter to export and fixt some debugging
Account for files not matching the sender/title pattern
Added a safety note
Wrong regex on the name parser
Renamed the command to something slightly less ambiguous
