Add a --mvn flag that builds with mvn package

[kmonkeyjam]

This assumes you have maven with an outputDir set for packaging that looks like gradle that takes an outputDir directory that is the temp directory generated by gordon. Not sure where to put this documentation:

<build>
    <outputDirectory>${outputDir}</outputDirectory>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.1</version>
        <configuration>
          <source>8</source>
          <target>8</target>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <executions>
          <execution>
            <id>copy-dependencies</id>
            <phase>prepare-package</phase>
            <goals>
              <goal>copy-dependencies</goal>
            </goals>
            <configuration>
              <outputDirectory>${outputDir}/lib</outputDirectory>
              <includeScope>runtime</includeScope>
              <overWriteReleases>false</overWriteReleases>
              <overWriteSnapshots>true</overWriteSnapshots>
              <overWriteIfNewer>true</overWriteIfNewer>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>

[kmonkeyjam]

Adam is going to make some modifications to this and re-submit a better pull request.