Adds ability to toggle system site package inclusion in runtime

[ipmb]

This makes the zipapps work a little more like virtualenv. It uses the same default as virtualenv (no system site packages) which is not backwards compatible. To retain the current functionality, you can build with the --system-site-packages flag or add SHIV_SYSTEM_SITE_PACKAGES=True during runtime.

Original issue: #65
Includes #64 since this is related

[coveralls]

Pull Request Test Coverage Report for Build 166

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.8%) to 77.972%

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Files with Coverage Reduction	New Missed Lines	%
.tox/py36/lib/python3.6/site-packages/shiv/cli.py	6	87.5%
.tox/py36/lib/python3.6/site-packages/shiv/bootstrap/init.py	10	63.77%

Totals
Change from base Build 159:	1.8%
Covered Lines:	223
Relevant Lines:	286

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💛 - Coveralls

[ipmb]

Just realized this doesn't account for removing paths added by .pth files. You'd probably need to do something like this as well https://github.com/pantsbuild/pex/blob/8f9c8ddfd4ac2cb0dc7ca566b392dc79f1fb324e/pex/pex.py#L262-L277