Need to support ID's in partial names

[machty]

Wait til after #75 is done, but need Emblem equivalent for handlebars-lang/handlebars.js#519

[machty]

@decasia This is all yours, should be pretty easy to do. Seems like you'll want to do something pretty similar to this PR: handlebars-lang/handlebars.js#526 I haven't looked too much into it, but should be decently straightforward. I'll help along the way if you have any questions.

[decasia]

Hi @machty, sorry to take two weeks to get back to this. But do you think you could take a quick glance at my first stab at this patch here? I added a unit test for the expanded issues, which currently passes, but there are still two outstanding issues:

1. It supports $ and . in partial names just fine, but it doesn't handle slashes correctly. I think Handlebars rewrites slashes in partial names as periods, but I don't know much about Handlebars internals so I'm not sure.
2. The larger issue: It looks like the Handlebars folks only very briefly handled this issue by updating the regex. The first accepted patch was this but the second one was (this)[https://github.com/wycats/handlebars.js/commit/da2aabe7bdc75e29178d920332fad9de183de8a0]. The second patch does away with the partial name regex altogether, and instead redefines partialName like so:

partialName
  : path { $$ = new yy.PartialNameNode($1); }
  | STRING { $$ = new yy.PartialNameNode(new yy.StringNode($1)); }
  | INTEGER { $$ = new yy.PartialNameNode(new yy.IntegerNode($1)); }
  ;

I assume we could do something similar in Emblem's legacyPartialName definition but I'm not familiar enough with PegJS grammar rules to quite understand how to write that. Can you give me any tips?