Image pulling

[MrMino]

Currently, when the image is not found by the docker daemon, dockernel install fails with a traceback, without even trying to search for the image on the web.

Make it convenient to download Dockernel images, without having to do any docker pull. Might be a separate subcommand - #2 proposed dockernel get. This would also make the subcommand issue the install() afterwards.

Dockernel images would get some priority on naming, so that people don't have to type dockernel get dockernelized/rust in order to get the Rust kernel, a simple dockernel get rust should suffice.

Some metadata might be stored with the image, so that things like --language can be populated automatically. It might be also a good idea to restrict pulling to images that contain some predefined "Dockernel-compatible" marker. With an override, of course.