Completion command for bash

[jvalkeal]

• 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 Add support for bash completion #343