Support for helm plugins

[ghost]

It would be good to have an option to execute arbitrary helm plugins.

for this we would need to be able to define plugins in the pom.xml similar to this:

<configuration>
    ...
    <plugins>
        <plugin>https://github.com/xyz/my-plugin</plugin>
    </plugins>
</configuration>
<executions>
    <execution>
        <goals>
            <goal>my-plugin</goal>
            <configuration>
              <options>
                <option>arg1</option>
                <option>arg2</option>
              </options>
            </configuration>
        </goals>
    </execution>
</executions>

[pvorb]

This would be really helpful. It should also be possible to specify the plugin version:

<configuration>
    ...
    <plugins>
        <plugin>
            <url>https://github.com/xyz/my-plugin</url>
            <version>master</version>
        </plugin>
    </plugins>
</configuration>

The <version> tag should be optional.

[ghost]

perhaps the tags should be named <helmPlugins>, <helmPlugin> to avoid confusion with the maven tags