Add `toggleClass` and `sectionClass` to options

 - Remove hardcoded '.readmore-js-section' and '.readmore-js-toggle' classes and replace them with extendable options
 - CSS Embed code updated to take variable class names into consideration
 - CSS Embed de-duplication code modified to handle multiple section and toggle class variations
 - 'sectionClass' variable in toggleSlider() renamed to 'sectionStateClass' to better reflect purpose and avoid clash with sectionClass in options
 - readmore.min.js rebuilt with uglifyjs
 - Updated README.md to enumerate the new options