Added basic Maven Plugin to validate mustache templates

This PR introduces a rudimentary plugin for the Apache Maven build tool to validate any mustache files found in the projects resources directory during the `process-resources` phase.

To enable, include the following in your projects pom.xml

    <plugin>
      <groupId>com.github.spullara.mustache.java</groupId>
      <artifactId>mustache-maven-plugin</artifactId>
      <version>0.8.16-SNAPSHOT</version>
      <executions>
        <execution>
          <id>validate</id>
          <goals>
            <goal>validate</goal>
          </goals>
        </execution>
      </executions>
    </plugin>