Add mainClass option for Repackage task

Add `mainClass` property option to the Repackage task. If the property
is defined within a task, it works in the same way as if it defined
within the springBoot{} 'ext' properties section.

Option is valid only for that specific task where it is defined, and
will override option defined in springBoot{} ext properties.

Fixes gh-283

spring-boot-tools/spring-boot-gradle-plugin/src/main/groovy/org/springframework/boot/gradle/task/Repackage.java

@@ -32,6 +32,7 @@ import org.springframework.boot.loader.tools.Repackager;
  * Repackage task.
  * 
  * @author Phillip Webb
+ * @author Janne Valkealahti
  */
 public class Repackage extends DefaultTask {
 
@@ -41,6 +42,8 @@ public class Repackage extends DefaultTask {
 
 	private Object withJarTask;
 
+	private String mainClass;
+
 	public void setCustomConfiguration(String customConfiguration) {
 		this.customConfiguration = customConfiguration;
 	}
@@ -49,6 +52,10 @@ public class Repackage extends DefaultTask {
 		this.withJarTask = withJarTask;
 	}
 
+	public void setMainClass(String mainClass) {
+		this.mainClass = mainClass;
+	}
+
 	@TaskAction
 	public void repackage() {
 		Project project = getProject();
@@ -64,6 +71,7 @@ public class Repackage extends DefaultTask {
 		else if (extension.getCustomConfiguration() != null) {
 			libraries.setCustomConfigurationName(extension.getCustomConfiguration());
 		}
+
 		project.getTasks().withType(Jar.class, new Action<Jar>() {
 
 			@Override
@@ -99,7 +107,9 @@ public class Repackage extends DefaultTask {
 								}
 							};
 						};
-						repackager.setMainClass(extension.getMainClass());
+						repackager
+								.setMainClass(Repackage.this.mainClass != null ? Repackage.this.mainClass
+										: extension.getMainClass());
 						if (extension.convertLayout() != null) {
 							repackager.setLayout(extension.convertLayout());
 						}