Add a collection option to 'addmedia' command line uploading functionnality. Specify the collection with its slug.

Use like this:

gmg addmedia user file.jpg -c collection-slug

​Here's the patch

This is my first contribution, please tell me what I did wrong :-)

Here is a small patch moving the datetime_format used in various places to the config. This allows for a global override per site.

I set the default in the config spec to what was previously used throughout the code, but I find it rather awkward. I think %c would be a better default.