Feature: EmacsFileVars as inline buildParser options

Trigger https://groups.google.com/d/topic/pegjs/Na7YWnz6Bmg/discussion

This PR is mostly a discussion placeholder.

* Allows for self-contained grammars (mostly due to being able to define `allowedStartRules`)

* EmacsFileVars act as defaults, so calling `peg.buildParser(grammar, options)` will override them

* Values can be any JSON value

* Why [EmacsFileVars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Specifying-File-Variables.html#Specifying-File-Variables)? - Why not? Is there something more "standardized"?