Add InitCommand to execute groovy script on startup

InitCommand runs on creation of SpringCli so it can search for additional
Commands in updated classpath. Also added as interactive command in Shell
session.

spring-boot-cli/src/test/resources/grab.groovy

@@ -0,0 +1,6 @@
+@GrabResolver(name="test", root="file:./src/test/plugins")
+@Grab("custom:custom:0.0.1")
+@Controller
+class Foo {}
+
+println "Hello Grab"

spring-boot-cli/src/test/resources/init.groovy

@@ -0,0 +1 @@
+println "Hello World"