[MJMOD-20] set jmod --main-class argument if it is set.

[andretadeu]

Hi!

This pull request sets the parameter --main-class if it was set in:

<plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jmod-plugin</artifactId>
                <configuration>
                    <mainClass>com.andretadeu.greetings.Main</mainClass>
                    <modulePath>target/jmods</modulePath>
                    <moduleVersion>${project.version}</moduleVersion>
                </configuration>
...
</plugin>

otherwise, if I am not able to set, I may not run jmod in modules that contains a Main class.

Thanks,

André Tadeu de Carvalho

[andretadeu]

@rfscholte Done.

[jira-importer]

Resolve #57

[jira-importer]

Resolve #57