Add CPAN skeleton

I've added a working CPAN skeleton. It doesn't care about versions of dependencies because CPAN seems to list minimum requirements, which I don't think conda supports (easily) yet.

I've also added support for attempting to recursively build recipes if there is a recipe corresponding to a missing package in your current directory. This has really come in handy when testing this out.

Similarly, there's a `--recursive` flag for the `skeleton cpan` command that will recursively create the recipes for dependencies.

In order to build any CPAN recipes, you will need the version of Perl I provide the recipe for [here](https://github.com/conda/conda-recipes/pull/91). It bundles [CPANminus](https://metacpan.org/pod/App::cpanminus) (aka `cpanm`), which is used for all the building.

You can see a whole repository full of recipes based on this skeleton [here](https://github.com/dan-blanchard/conda-cpan-recipes).

Please let me know if there's anything that needs to change.