doc: change readme to Markdown and add more information
doc: added sample configuration files

Closes #12
commands: add support for setting the prefix in the CLI

change f2c626f introduced support for setting the prefix for each of the
image lists. This change makes possible to set it using the CLI.
image list: Catch IOError exception

Catch the IOError exception and reraise our own exceptio so that the
traceback does not appear in the output.