Add skip_core_modules Option To Skip perl Core Modules

This pull provides a boolean `skip_core_modules` option. When set to a true value, modules that are in perl core are skipped. This prevents the annoying prompts to install things like `ExtUtils::MakeMaker` or `IPC::Open3` without requiring a whole bunch of `skip` options. The rationale for skipping those modules is that when the dist is built on not the latest perl (5.14 in my case), I can't install the latest `ExtUtils::MakeMaker` or `IPC::Open3` anyway, as those require newer perl—the `skip_core_modules` option set to a true values would allow me to skip those modules.