Check-jsonschema

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- Bugfix for package metadata to include builtin schemas

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- ``check-jsonschema`` now ships with vendored versions of the external schemas
used for the default suite of hooks. The vendored schemas are used as a
failover option in the event that downloading an external schema fails. This
resolves :issue:`21`
- New CLI options, ``--builtin-schema`` and ``--failover-builtin-schema`` are
available to access the builtin schemas. See documentation for details.
- Use the latest version (version 4) of the ``jsonschema`` library. Note
that ``jsonschema`` has dropped support for python3.6, and ``check-jsonschema``
will therefore use ``jsonschema`` version 3 when running on python3.6
- The path shown in error messages is now a valid
`JSONPath <https://goessner.net/articles/JsonPath/>`_ expression

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- Bugfix: validation errors were not being displayed correctly.
- Errors are now sent to stderr instead of stdout.

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- Exception tracebacks for several known-cases are printed in a shortened
format. A new option, ``--traceback-mode`` can be used to request long traces,
as in ``--traceback-mode full``
- For schemas which do not include ``$id``, the schema URI will be used for
``$ref`` resolution. This applies to HTTP(S) schema URI as well as to local
paths. Thanks to :user:`dkolepp` for the bug report and contributions!

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- Add support for string format verification, by enabling use of the
``jsonschema.FormatChecker``. This is enabled by default, but can be disabled
with the ``--disable-format`` flag

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- Improved error output when the schema itself is invalid, either because it is
not JSON or because it does not validate under its relevant metaschema