[proguard-maven-plugin] Improvement: Add possibility to add all dependencies as -injars

[Dmitry Katsubo]

Currently only one dependency (for current project) is passed as "-injars". Thus one need to use maven-assembly-plugin to prepare all-in-one jar and then use that artifact using

<configuration>
    <injar>...</injar>
</configuration>

Would be nice if there is an additional option to pull all dependencies to "-injars" instead of "-libraryjars".