gallery: Add per-image caption/title support.

To add caption/title to a picture, in your article simply add a
'gallerycaptions' tag in your metadata, whose value is a dictionnary of
(filename, title) tuples.

Example:
gallery: <Gallery Name>
gallerycaptions: { 'image_name1': 'caption1', 'image_name4': 'caption4', }

Then you will be able to use article.gallerycaptions.get('image_nameX')
in the templates, to retrieve the title associated to the image.