Add support of shorthand property syntax

[ericleib]

This Pull Request adds support for the shorthand property notation.

This allows to simplify an expression from:

{ "a": a } 

to:

{ a } 

This also works for variables:

( $foo:= 42; {$foo} ) 

output:

{ "foo": 42 } 

[mattbaileyuk]

I believe it's quite a Javascript-y syntax concept, but one I do find convenient, and the implementation is not tied to Javascript, so I've got nothing against including this 🙂

Thoughts, @andrew-coleman?