Add support for arbitrary rule metadata.

[igordejanovic]

This could be used in recognizers, actions, disambituation/error recovery callbacks, error reporting etc. to extend the semantics of parglare grammar language.

Ideas for the syntax (label is meta-data attached to my_rule grammar rule):

<label: "My Rule">
my_rule: ... ;

or just extending currently build-in meta-data in {}:

my_rule: ... {label:"My Rule"};

Once defined it could be accessed on grammar symbol as an attribute for example.

I like more the later syntax approach as it wouldn't introduce additional syntax noise and with #17 it would enable defining meta-data per productions also without change in syntax.

[igordejanovic]

Done on master branch.

Syntax is different from the initially proposed. Please see the docs.