Enhancement - Add `toggleClass` and `sectionClass` to options

#### Use cases
* Ability to avoid conflicts with existing `.readmore-js-section` and `.readmore-js-toggle` classes.
* Including Readmore.js within a project with strict class naming rules, could be BEM, OOCSS, or otherwise.

#### Changes 
* Removes hardcoded `.readmore-js-section` and `.readmore-js-toggle` classes and replaces 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
 * Renames `sectionClass` variable in `toggleSlider()` function to `sectionStateClass` to better reflect purpose and avoid clash with `sectionClass` option
 * `readmore.min.js` rebuilt with `uglifyjs -m`
 * Updated README.md to enumerate the new options