Snaps don't support bash autocompletion

Snaps have no way of installing files in /etc/bash_completion.d/ so that traditional bash auto-completion would be supported for installed snaps, particularly under classic. Snappy should provide for some means for a snap package to define auto-completion for it's commands, and snappy itself should support auto-completion similar to how it operates in ubuntu's default shell.

Regardless of how it's implemented, snaps need to support the idea of being able to tab complete commands within it's default shell.