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-standard/pom.xml

@@ -18,6 +18,10 @@
 			<groupId>org.springframework.shell</groupId>
 			<artifactId>spring-shell-core</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.antlr</groupId>
+			<artifactId>ST4</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>