Add SpringApplicationBuilder

Builder for SpringApplication and ApplicationContext instances with
convenient fluent API and context hierarchy support. Simple example
of a context hierarchy:

   new SpringApplicationBuilder(ParentConfig.class)
               .child(ChildConfig.class).run(args);

Another common use case is setting default arguments, e.g.
active Spring profiles, to set up the environment for an application:

     new SpringApplicationBuilder(Application.class).profiles("server")
 		.defaultArgs("--transport=local").run(args);

If your needs are simpler, consider using the static convenience
methods in SpringApplication instead.

[#49703716] [bs-116] Parent context for some beans maybe?

spring-boot-samples/spring-boot-sample-tomcat/src/test/java/org/springframework/boot/sample/tomcat/NonAutoConfigurationSampleTomcatApplicationTests.java

@@ -27,6 +27,7 @@ import org.junit.BeforeClass;
 import org.junit.Test;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration;
+import org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration;
 import org.springframework.boot.sample.tomcat.service.HelloWorldService;
@@ -54,7 +55,8 @@ public class NonAutoConfigurationSampleTomcatApplicationTests {
 
 	@Configuration
 	@Import({ EmbeddedServletContainerAutoConfiguration.class,
-			WebMvcAutoConfiguration.class, PropertyPlaceholderAutoConfiguration.class })
+			DispatcherServletAutoConfiguration.class, WebMvcAutoConfiguration.class,
+			PropertyPlaceholderAutoConfiguration.class })
 	@ComponentScan(basePackageClasses = { SampleController.class, HelloWorldService.class })
 	public static class NonAutoConfigurationSampleTomcatApplication {