Add support for anttask p2.mirror slicingOptions

[hacki11]

https://wiki.eclipse.org/Equinox/p2/Ant_Tasks#SlicingOptions
e.g. setting 'latestVersionOnly' flag to reduce multiple versions for single dependencies

usage example:

p2AsMaven {
    group 'eclipse-deps', {
        repoEclipse '3.8.2'
        repo 'http://download.eclipse.org/releases/juno/'
        iu 'org.eclipse.core.jobs'
        slicingOption 'latestVersionOnly', 'true'
        repo2runnable()
    }
}

[nedtwigg]

Thanks for the contribution! I added attribution and docs here: 261de95

Would you like any changes?

[nedtwigg]

Released in 3.9.0.