Allow an alternative Esprima module to be used for parsing

[benjamn]

In other words, support the following option:

require("regenerator")(source, {
  esprima: require("custom-esprima")
});

Right now Regenerator always use the harmony branch, so any custom Esprima implementation should probably be based on that branch (or at least have the same support for generator functions and yield expressions).

This will allow async functions and await expressions to be implemented as a separate package that depends on this Esprima branch: https://github.com/ariya/esprima/pull/234