add support for separate lexfile; update jison dependency

This pull request adds support for separate Jison lexfile, supplied by second source file (analogous to Jison cli behavior).  In order to support this I had to pull in a couple of functions (and Jison dependencies) from the Jison cli (Jison is MIT licensed too, this should be ok).  This is not ideal and I plan on working on a separate pull request for Jison upstream so that the Generator constructor API can be enhanced to include this functionality without having to perform it in the client.  I updated the Jison dependency since it was rather out of date.  I preserved the spirit of the test that was in place, and added a case for separate grammar and lexfile.