Select multiple

When selecting a mod with several conflicting subdependencies, the GUI will ask the user to select one and continue, the CLI does not do this. This adds support for selecting 1 item from a list of items, and supports a preselected default. This add the CLI support for using the interface in PR https://github.com/KSP-CKAN/CKAN-core/pull/20 for selecting one mod from a list of mods, and selecting the default KSP instance.

This is part of the old PR at https://github.com/KSP-CKAN/CKAN/pull/586, and should solve #1.