Make Push Option configurable

[sparsick]

As developer, I want to set push option depending on my Git Management System. The configurable push options should be enabled for native Git and JGit.

The push option has to be set manually by the developer in the configuration section of the plugin.

[georgberky]

We have agreed on the following configuration format:

    <build>
        <plugins>
            <plugin>
                <groupId>${project.groupId}</groupId>
                <artifactId>${project.artifactId}</artifactId>
                <version>${project.version}</version>
                <configuration>
                    <gitProvider>JGIT</gitProvider>
                    <gitOptions>
                        <pushOptions>
                            <foo>bar</foo>
                        </pushOptions>
                    </gitOptions>
                </configuration>
            </plugin>
        </plugins>
    </build>