Plugin configuration to support multi-licensing scenarios

[adamretter]

This adds support for source-code projects that use multi-licensing (e.g. dual-licensing).

If your source code makes use of multi-licensing, then instead of
a <header> or <inlineHeader> element in the configuration
you can use a <multi> element.

The <multi> element allows you to specify an optional preamble,
one or more header (or inlineHeader) and separators between them. These
options are concatenated together to produce a header template.

    <plugin>
        <groupId>com.mycila</groupId>
        <artifactId>license-maven-plugin</artifactId>
        <version>X.Y.ga</version>
        <configuration>
            <multi>
                <preamble><![CDATA[This product is dual-licensed under both the GPLv2 and Apache 2.0 License.]]></preamble>
                <header>GPL-2.txt</header>
                <separator>======================================================================</separator>
                <header>com/mycila/maven/plugin/license/templates/APACHE-2.txt</header>
            </multi>
            <properties>
                <owner>Mycila</owner>
                <email>mathieu.carbou@gmail.com</email>
            </properties>
            <excludes>
                <exclude>**/README</exclude>
                <exclude>src/test/resources/**</exclude>
                <exclude>src/main/resources/**</exclude>
            </excludes>
        </configuration>
        <executions>
            <execution>
                <goals>
                    <goal>check</goal>
                </goals>
            </execution>
        </executions>
    </plugin>

[mathieucarbou]

didn' try the feature, didn't deeply look at the changes but this is a cool addition and they seem pretty much addition. thanks!