Add TOC collapse option to pressbooks-book (Luther) theme
Improve efficiency in toc_collapse by caching jQuery(this).find(".dashicons")
Merge pull request #86 from drlippman/toc_collapse

Add TOC collapse option to pressbooks-book (Luther) theme.
Adjusted descriptive text for #86.
Support for multilevel TOC in webbooks (#1047).
Support for multilevel TOC in PDF front- and back-matter (#1047).
Support for multilevel TOC in EPUB and HPUB front- and back-matter (#1047).
Renamed subsection parsing functions as they no longer just apply to chapters.