Add option to prevent truncation of tags.

[TZdyrski]

IMPORTANT: Do not merge until next release of ExifRead after 2.1.2. This relies on a new option, truncate_tags, being passed to exifread.process_file.

Add option to prevent EXIF tag data from being truncated. Currently, exifread was truncating EXIF tag data to 20 characters and appending an ellipsis, ... . This ellipsis would then throw an error. To resolve this, set the option truncate_tag=false (default true) in the ini file to prevent this truncation and resolve the error.