Add loader.args to PropertiesLauncher

Also new section of README for propery keys used by the
launcher.

spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/PropertiesLauncherTests.java

@@ -20,6 +20,7 @@ import java.io.File;
 import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
+import java.util.Arrays;
 import java.util.Collections;
 
 import org.junit.After;
@@ -147,6 +148,13 @@ public class PropertiesLauncherTests {
 		assertEquals("demo.Application", System.getProperty("loader.main"));
 	}
 
+	@Test
+	public void testArgsEnhanced() throws Exception {
+		System.setProperty("loader.args", "foo");
+		PropertiesLauncher launcher = new PropertiesLauncher();
+		assertEquals("[foo, bar]", Arrays.asList(launcher.getArgs("bar")).toString());
+	}
+
 	private void waitFor(String value) throws Exception {
 		int count = 0;
 		boolean timeout = false;