Check-jsonschema

-----

- ``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

-----

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

-----

- 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!

-----

- 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

-----

- 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

-----

- Added the ``--default-filetype`` flag, which sets a default of JSON or YAML
loading to use when ``identify`` does not detect the filetype of an instance
file. Defaults to failure on extensionless files.
- Schemafiles are now passed through ``os.path.expanduser``, meaning that a
schema path of ``~/myschema.json`` will be expanded by check-jsonschema
itself (:issue:`9`)
- Performance enhancement for testing many files: only load the schema once
- Added ``--no-cache`` option to disable schema caching
- Change the default schema download cache directory from
``jsonschema_validate`` to ``check_jsonschema/downloads``.
e.g. ``~/.cache/jsonschema_validate`` is now
``~/.cache/check_jsonschema/downloads``.
Caches will now be in the following locations for different platforms
and environments:

- ``$XDG_CACHE_HOME/check_jsonschema/downloads`` (Linux/other, XDG cache dir)
- ``~/.cache/check_jsonschema/downloads`` (Linux/other, no XDG cache dir set)
- ``~/Library/Caches/check_jsonschema/downloads`` (macOS)
- ``%LOCALAPPDATA%\check_jsonschema\downloads`` (Windows, local app data set)
- ``%APPDATA%\check_jsonschema\downloads`` (Windows, no local app data set, but appdata set)