Implement minimal EXIF and IPTC tag support

[sebastiaan-lampo]

Updated version of PR #32.

Rebased on the updated version of HPS. I've added tests to cover the EXIF and IPTC additions. Minor changes to the configuration tests are required as this PR introduces 3 potential new config file entries. These have been added to the config tests.

This feature allows:

• Back filling the copyright or caption data from an EXIF or IPTC metadata field if not defined in the album.yml
• Prepare for allowing access to all tags in the shortcode by making (a subset of) tags available as a property of the Photo object