* Add suppport for ES6 style exports

Adds support for ECMAScript 6 module exports during Javascript
generation. This is useful as many development projects are
switching to this new standard and away from custom module
solutions. By integrating support into flatbuffers, users
do not need to perform additional post-processing of generated
files in order to use flatbuffers output directly in their
codebases.

Reference to ECMAScript 6 modules:
https://www.ecma-international.org/ecma-262/6.0/#sec-exports

Changes:
* Added `--es6-js-export` option to cli parser tool
* Added conditional code to generate a ES6 style export
  line, replacing the normal NodeJS/RequireJS line.

* Fixed missing export statements

Added exports for definition and struct names that were not inside namespaces

* Updated Compiler.md with new generator option

Added entry to Compiler.md in docs for the `--es6-js-export` flag, including a brief description of the effects and usefulness.