Add gruntArgs parameter

Hi,

First of all, thanks for sharing this plugin.

When building with Node v0.10.10, the plugin fails because of the following Grunt warning:

   Warning: Arguments to path.join must be strings Use --force to continue.

While probably not ideal (I'm new to Yeoman/Bower/Grunt), I needed a way to pass the force argument, so I thought it might be useful to be able to set this as a configuration parameter.  With this patch, you'll be able to do the following:

            <plugin>
                <groupId>com.github.trecloux</groupId>
                <artifactId>yeoman-maven-plugin</artifactId>
                <version>0.2-SNAPSHOT</version>
                <configuration>
                    <gruntArgs>--no-color --force</gruntArgs>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>build</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

Leaving the parameter out will default to "--no-color".

I hope this is helpful.  Feel free to reject or modify as you see fit.  Thanks.