Support <springProfile> in logback configurations

Include support for a new <springProfile> element which can be used in
`logback-spring.xml` files to selectively enable or disable parts of the
configuration. For example:

	<configuration>
		...
		<springProfile name="staging">
			<logger name="sample.logback" level="TRACE" />
		</springProfile>
		...
	</configuration>

Fixes gh-3338

spring-boot-samples/spring-boot-sample-logback/src/test/java/sample/logback/SampleLogbackApplicationTests.java

@@ -35,4 +35,12 @@ public class SampleLogbackApplicationTests {
 		this.outputCapture.expect(not(containsString("Sample Trace Message")));
 	}
 
+	@Test
+	public void testProfile() throws Exception {
+		SampleLogbackApplication
+				.main(new String[] { "--spring.profiles.active=staging" });
+		this.outputCapture.expect(containsString("Sample Debug Message"));
+		this.outputCapture.expect(containsString("Sample Trace Message"));
+	}
+
 }