adding ability to override traceur options

- removing blockBinding (let) from default options since current
  implementation is not according to ES6 spec
- supporting to override options via es6ify.traceurOverrides
- adapting tests and adding test that shows that let is no longer
  supported by default, but that it is via an override
updating jsdoc and readme to new API

- documented and provided examples for traceurOverrides option
- updated supported vs. experimental features
1.0.0 traceur upgrade, experimental features disabled, traceur options overridable