Provide functionality to generate a basic metapackage

[torsava]

Originally reported by: Slavek Kabrda (Bitbucket: bkabrda, GitHub: bkabrda)

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• Bitbucket: https://bitbucket.org/bkabrda/spec2scl/issue/13

[torsava]

Original comment by Robert Kuska (Bitbucket: xkuska, GitHub: Unknown):

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Add alpha support for generating specfiles for metapackages, --meta-specfile, fixes #13