Add a "default specific" platform tag option for pure Python wheels

[agronholm]

Originally reported by: Nate Coraor (Bitbucket: natefoo, GitHub: natefoo)

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As detailed in issue #128 and issue #152, it's sometimes desirable to have a platform-specific platform tag even on pure Python wheels. This is now possible with the --plat-tag option, but this requires the user to know what the correct tag is for the build platform. This information is easily acquired from python setup.py bdist_wheel --help or by calling distutils.util.get_platform(), but it'd be more convenient and less error-prone if there was a way to specify "I want the default platform-specific tag on this pure python wheel."

This could be done in a few different ways:

• A special reserved platform tag passed to --plat-tag like default, arch_default, platform_default, platform, etc.
• A new option such as --nonpure-plat-tag

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• Bitbucket: https://bitbucket.org/pypa/wheel/issue/153

[ncoghlan]

Perhaps --plat-tag :default:?

pip already uses the surrounding colon convention to denote special values like --no-binary :all: and --only-binary :all:.