Update multiselect hash to require key names

[jamesls]

Instead of "{foo,bar}" you need to say "{foo: foo, bar: bar}". This makes it easier when you have more complicated non branched expressions, for example: "{foo: foo.bar.baz[0], first_state: states[0].name}"

This also exposed a bug in the lexer regarding double quoted identifiers and being able to escape quoted identifiers. I've added additional tests to capture this bug.

cc @garnaat @mtdowling

[mtdowling]

Looks good. Are you going to update the grammar in the readme too?

[jamesls]

Yeah, I just folded that into the docs PR, since I'm keeping a copy of the grammar in the sphinx docs: https://github.com/boto/jmespath/pull/14

[garnaat]

LGTM