Added support for sub-sections

This PR adds support for parsing sub-sections within each section. Just as with sections, sub-sections are defined with a heading tag in the settings options (`subSections`) and are set to `false` by default. The plugin also formats the content of the article to wrap each section in a `<section>` and each sub-section in a `<div>` for easier filtering and control of the article contents (and to set up an upcoming fix for "active" states).

NOTES: Naming conventions were updated to standardize options, ids and classes (classes now follow BEM naming). This PR will most likely break existing initializations and stylesheets.