Allowing variable literals prefixed by %

[ceymard]

This is about the ticket #36

It allows one to write things such as this ;

{
    var jinjs = { open: '{%', close: '%}' };
}

tag
   = %jinjs.open inside_tag %jinjs.close { ... }

It gets particularly useful with the options pull request.

[dmajda]

This pull request is related to #36, which I'd like to look at in detail after releasing PEG.js 1.0.0. This point is at least several months in the future and it doesn't make sense for this pull request to remain open until then. Closing.