[feature request]: Register mime types to open images with swappy

[nwg-piotr]

In addition to opening a file with swappy -f "~/Desktop/my-gnome-saved-file.png", which is great, you could also create a /usr/share/applications/swappy.desktop entry, and register opening png / jpg files, e.g. like this:

MimeType=image/jpeg;image/jpg;image/png;

[jtheoof]

So I added a desktop file with #41. If you're on Arch, can you install the latest from swappy-git and let me know if this is what you expected?

[nwg-piotr]

Works perfectly well. It would be even better if it also could open jpg one day. :) Not that it's super important.