Add macros to support Maven focused guides

Author: royclarkson

build_system_intro_maven.adoc

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+
+:linkattrs:
+
+First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with https://maven.apache.org[Maven] is included here. If you're not familiar with Maven, refer to link:/guides/gs/maven[Building Java Projects with Maven].

prereq_editor_jdk_maven_buildtools.adoc

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+:linkattrs:
+
+ifndef::java_version[:java_version: 1.6]
+
+ - About 15 minutes
+ - A favorite text editor or IDE
+ - http://www.oracle.com/technetwork/java/javase/downloads/index.html[JDK {java_version}] or later
+ - http://maven.apache.org/download.cgi[Maven 3.0+]
+ - You can also import the code from this guide as well as view the web page directly into link:/guides/gs/sts[Spring Tool Suite (STS)] and work your way through it from there.

run_the_application_with_maven.adoc

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+:link_attrs:
+
+ifndef::module[:module: service]
+
+== Run the {module}
+Run your {module} at the command line:
+
+[subs="attributes", role="has-copy-button"]
+....
+mvn spring-boot:run
+....

spring-boot-maven-plugin.adoc

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+The https://github.com/spring-projects/spring-boot/tree/master/spring-boot-tools/spring-boot-maven-plugin[Spring Boot Maven plugin] provides many convenient features:
+
+- It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.
+- It searches for the `public static void main()` method to flag as a runnable class. 
+- It provides a built-in dependency resolver that sets the version number to match https://github.com/spring-projects/spring-boot/blob/master/spring-boot-dependencies/pom.xml[Spring Boot dependencies]. You can override any version you wish, but it will default to Boot's chosen set of versions.
+