- Add basic support of defining a command `completion bash` which
outputs a generic bash script which can be used in a user environment.
- Idea for completion is copied from go's cobra library what comes for
a bash dance itself.
- Goes through command registry, builds a model for command structure
and uses antlr st4 for templating bash.
- Should give foundation to create other completions just like in cobra.
- Currently as we don't know a root-command in a generic way, option
`spring.shell.command.completion.root-command` is required user to set.
- Fixes#343
668ddb458e