Strip-hints

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Consolidated the functional interface function ``strip_file_to_string`` so it
now accepts ``only_test_for_changes`` as an argument.

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Now handles annotated expressions of the form ``d["key"]: int`` and ``d["key"]: int == 4``.

Add an option ``--only-test-for-changes`` to just test for changes.

Fixed a nasty bug introduced in last version, keywords like "try" mistaken for
annotated variables.

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Now handles annotated dotted-access assignments (a simple form of annotated
expressions). For example, ``x.y.z : int``.

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New Features:

* The new ``strip_on_import`` function allows stripping Python 2 files on import.

Bug Fixes: None.

Other changes:

* Expanded README documentation.

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Initial release.