JMESPath query support

[ssato]

Maybe anyconfig can support to query data with JMESPath expressions.

• JMESPath: http://jmespath.org
• Python library for JMESPath: https://github.com/jmespath/jmespath.py

[ssato]

Done partially with the commits 357cdd5, 507f3d10 and so on.

• Usage examples of ac_query for the 'load' API: https://github.com/ssato/python-anyconfig/blob/master/tests/api.py#L684
• Usage examples of the CLI frontend: http://python-anyconfig.readthedocs.io/en/latest/cli.html#query-get-set-extract-or-set-part-of-input-config

[ssato]

LGTM. Let it close once