Ebookmaker

- fixed minor issue where "too deep" errors were emitted for self-links. Thanks to rfrank for the error report.
- fixed deprecation warning from Docutils; should be ok for Docutils > 0.1

- SVG files are now considered images and included in EPUBs. They were being discarded. SVG files are not scaled or compressed by ebookmaker - the renderer should be able to auto-scale. This appears to fix kindlegen failures associated with svg images.
- fixed the rst test

- changed font for rst conversion. Linux Libertine was unmaintained since 2012 and no package was available for CENTOS8. We switched to the closest replacement, Libertinus https://github.com/alerque/libertinus
- added documentation about configuration for rst conversion
- the deprecated 'handheld' media query was being used to prevent ebookmaker from stripping floats. to preserve this feature, ebookmaker no longer strips floats when the css selector contains the x-ebookmaker class. Most likely, float stripping was originally needed because html pages were designed before the advent of EPUB. Today, we can assume that if the html designer uses the x-ebookmaker class, they've considered the impact of the float on the generated EPUB.
- assorted delinting

- added 'x-ebookmaker' class to epub body elements. There are now 4 "x-ebookmaker" classes
- css can now apply styles that are triggered by being a descendent of .x-ebookmaker. This addition is meant to replace the 'handheld' media query that is deprecated in HTML 5
- the 'x-ebookmaker-important' class on on image element tells ebookmaker not to remove the image, even in no-images builds.
- the 'x-ebookmaker-drop' class tells ebookmaker to remove an element and its descendents from ebook builds.
- the 'x-ebookmaker-pageno' class is applied to some span elements whose content has been stripped because they use a class that indicates they represent page numbers: pagenum pageno page pb folionum foliono
- added mayan as a language not supported by kindlegen
- typos fixed in README - thanks Joseph Koshy

- fixed minor issue where the spider was getting confused when iterating on WrapperParsers.

- handle invalid quantization table in jpeg files when using quality 'keep'
- respect rel="nofollow" attribute: this allows authors to link to an alternate version file in html without duplicating content in the EPUB file.
- set wrappers to nonlinear in spine.
- fixed bugs and ugliness in toc generation
- when the same header level is consecutive, only one toc item is generated. (We see this used to make multiline heads or titles)
- toc normalization made a hash of the toc. Now the toc is normalized in the epubwriter, not in the parser.
- add display:block to standard css sheet to prevent hidden headings from breaking kindlegen
- added a configuration option to use calibre (or whatever!) for non-supported in kindleget languages.