Add support of includePluginDependencies for the exec goal

[astefanutti]

In some use cases, that'd be useful to add some dependencies to the classpath for the exec goal. For example, one could write:

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>exec-maven-plugin</artifactId>
    <configuration>
        <executable>java</executable>
        <commandlineArgs>-classpath %classpath org.apache.camel.cdi.Main</commandlineArgs>
        <includePluginDependencies>true</includePluginDependencies>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>io.astefanutti.camel.cdi</groupId>
            <artifactId>camel-cdi</artifactId>
            <version>${project.version}</version>
        </dependency>
    </dependencies>
</plugin>

So that the additional dependency is contained in the %classpath variable.

[bondolo]

A feature is necessary for adding additional dependencies but using pluginDependencies is a hack.

[sebthom]

@slawekjaranowski I think this issue can be closed now that #432 is merged.