Implement JEP 1

This required updating the grammar, but no code changes were needed.
I've updated the compliance tests with the examples from the JEP.

Note that there was one example that was changed from the original
spec.  The OR expression example was changed because a wildcard
on a list type will never evaluate to ``null``, and therefore will
always be selected if it's the first expression in an OR expression.
Merge branch 'jep-1' into develop

* jep-1:
  Implement JEP 1
Update spec/tests for wildcard syntax

This clarifies what happens for wildcard projections on
non array/object types.  The compliance tests are also updated
to test what's in the spec with respect to non list/object types.
Merge branch 'doc-updates' into develop

* doc-updates:
  Update spec/tests for wildcard syntax
Add JEP 6 proposal
Update JEP to allow "_" as a valid unquoted identifier
Add note about compliance test differences
Implement JEP 6
Add unicode literal compliance test
Update spec with the JEP 6 grammar

Also removed the grammar from the README and replaced with a link.
Must people looking at the README don't care about the actual
grammar (other than to know that there is one).  For those that
are interested, they can follow the link to get to the grammar.
Also avoids us having to keep the grammar in two places.  Now the
official grammar lives in docs/specification.rst.
Remove duplicate test case
Merge branch 'jep-6' into develop

* jep-6:
  Remove duplicate test case
  Update spec with the JEP 6 grammar
  Add unicode literal compliance test
  Implement JEP 6
  Add note about compliance test differences
  Update JEP to allow "_" as a valid unquoted identifier
  Add JEP 6 proposal
Don't project non list elements in flattening operator

There was a similar fix for wildcards.  The flattening operator
needs this behavior as well.
Merge branch 'fix-flattening-none' into develop

* fix-flattening-none:
  Don't project non list elements in flattening operator
Bump version to 0.2.1
Merge branch 'release-0.2.1' into develop

* release-0.2.1:
  Bump version to 0.2.1
Add copyright to license file

Fixes #36
Update grammar with new filter expressions
Update perftest to measure lexing time
Allow a single compliance test to be run via env vars