Add an option to log banner rather than printing it to standard out

This commit adds the option to output the banner using the logger
instead of standard out. Rather than taking a boolean
spring.main.show-banner is now configured using an enum. Three values
are supported:

 - LOG: the banner is logged
 - CONSOLE: the banner is printed to standard out (previously true)
 - OFF: the banner is switched off (previously false)

The default behavior remains unchanged; the banner will be printed to
standard out.

Closes gh-4022
See gh-4001

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -123,7 +123,7 @@ might have.
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	spring.main.web_environment=false
-	spring.main.show_banner=false
+	spring.main.show_banner=OFF
 ----
 
 and then the Spring Boot banner will not be printed on startup, and the application will
@@ -139,7 +139,7 @@ consider this application
 [source,java,indent=0]
 ----
 	new SpringApplicationBuilder()
-		.showBanner(false)
+		.showBanner(Banner.Mode.OFF)
 		.sources(demo.MyApp.class)
 		.run(args);
 ----
@@ -149,7 +149,7 @@ used with the following configuration:
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	spring.main.sources=com.acme.Config,com.acme.ExtraConfig
-	spring.main.show-banner=true
+	spring.main.show-banner=CONSOLE
 ----
 
 The actual application will _now_ show the banner (as overridden by configuration) and use

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -124,7 +124,7 @@ For example:
 [source,java,indent=0]
 ----
 	new SpringApplicationBuilder()
-		.showBanner(false)
+		.showBanner(Banner.Mode.OFF)
 		.sources(Parent.class)
 		.child(Application.class)
 		.run(args);