Initial implementation of Allowing Unknown Images directory naming

[unapproachable]

Initial implementation of #74

Allows user to specify where Unknown date files go via a command line parameter. Parameter --no-date-dir overrides the default of "unknown".

Usage:

phockup INPUTDIR OUTPUTDIR --no-date-dir=UNKNOWNDIR

Example usage:

phockup ~/Pictures/camera ~/Pictures/sorted --no-date-dir=Miscellaneous

This would put all files without valid EXIF date information in the ~/Pictures/sorted/Miscellaneous directory.

Also included:

• Added pytest use case named test_no_exif_directory for alternative unknown directory name testing.
• Refactored phockup constants to the Phockup class ease of scoping for imports and tight coupling to the class.
  DEFAULT_DIR_FORMAT = ['%Y', '%m', '%d']
DEFAULT_NO_DATE_DIRECTORY = "unknown"
• Implemented the existing DEFAULT_DIR_FORMAT constant as the fallback case for the --d\--date command line parameter

[unapproachable]

I can't/won't correct the Readme violating Flake8 validation. Is there a way to prevent it from being scanned as part of the linting process?