Add custom HTML template for caption output

[arnowelzel]

Add an option to provide a custom HTML template for the caption output with placeholders similar to custom sharing links:

{{caption}} - The image caption
{{exif-camera}} - EXIF information, camera model
{{exif-focal}} - EXIF information, focal length
{{exif-fstop}} - EXIF information, f-stop
{{exif-shutter}} - EXIF information, shutter speed
{{exif-iso}} - EXIF information, ISO
{{exif-date}} - EXIF information, date/time

All EXIF elements would be enclosed in <span> elements as documented to keep compatibility with existing CSS customization (e.h. <span class="pspw__caption__exif_focal">...</span> for {{exif-focal}}.

If no custom template is used, the default format would be:

{{caption}}<div class="pspw__caption__exif">{{exif-camera}} ({{exif-focal}}, {{exif-fstop}}, {{exif-shutter}}, {{exif-iso}}, {{exif-date}})</div>