Completion command for bash

- 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

spring-shell-samples/src/main/resources/application.yml

@@ -1,3 +1,10 @@
 logging:
   level:
    root: error
+spring:
+  main:
+    banner-mode: off
+  shell:
+    command:
+      completion:
+        root-command: spring-shell-samples