Replace Grunt with npm scripts, and rewrite mkdoc.py in JS

[kirsle]

Relevant reading: Why I Left Gulp and Grunt for npm Scripts

I'd like to be able to drop Grunt as a dev dependency and just use simple npm scripts for all the jobs that Grunt currently is doing (build, minify, unit tests, etc.)

Furthermore, the documentation generator tool, mkdoc.py is written in Python to render the Markdown and HTML files in the docs/ folder. This should be re-written in JavaScript so that other contributors can be expected to rebuild the docs when they change the source files in their pull requests.

While I'm at it, the mkdoc.py should not generate documentation for private functions in the docs/rivescript.md output file. Even though I tell developers not to call the private functions, they do anyway, and their visibility in the public documentation is probably to blame for that.