Support setting location within image description for usage with show_map

[mika]

I'm using the following configuration in sigal.conf.py:

show_map = True
leaflet_provider = 'OpenStreetMap.Mapnik'

I've got a collection of pictures where only some of them include GPS information inside their EXIF data/tags. Now I can use something like:

Location: 46.994046,15.440132

or:

Location: Graz Airport

inside the description of an image (since I know where this image was taken), though it's only displayed in the info section of the sigal gallery and AFAICT not considered for displaying on the map view (via key m).

Would be great if there'd be a way how to add location/GPS information to pictures which don't have the according information inside their EXIF data and use that for the show_map feature then.

Thanks for sigal, it's great! :+1:

[saimn]

Setting the coordinates in the image metadata is a good idea! Support names instead of coordinates could be done with some additional dependency (geopy ?), I guess this should be optional.