Support label with @Option

- Add `label` field into `@Option`. This sets option label if
  `label` field has any text.
- Fixes #732

spring-shell-core/src/main/java/org/springframework/shell/command/annotation/Option.java

@@ -70,6 +70,13 @@ public @interface Option {
 	 */
 	String description() default "";
 
+	/**
+	 * Return a label of the option.
+	 *
+	 * @return label of the option
+	 */
+	String label() default "";
+
 	/**
 	 * Define option arity.
 	 *

spring-shell-core/src/main/java/org/springframework/shell/command/annotation/support/CommandRegistrationFactoryBean.java

@@ -251,6 +251,9 @@ class CommandRegistrationFactoryBean implements FactoryBean<CommandRegistration>
 				optionSpec.shortNames(shortNames.toArray(new Character[0]));
 				optionSpec.position(mp.getParameterIndex());
 				optionSpec.description(so.description());
+				if (StringUtils.hasText(so.label())) {
+					optionSpec.label(so.label());
+				}
 				int arityMin = so.arityMin();
 				int arityMax = so.arityMax();
 				if (arityMin > -1) {