Ebookmaker

- clean up txt boilerplate. fixes easy parts of 220
- refactor `enclose_text()` to properly deal with html4 transitional files
- don't allow non-images as img src
- libgutenberg 0.10.22 (fixes 508 attribute issue)

- fixed an issue parsing large text files. "The Entire Project Gutenberg Works of Mark Twain" was 1.2M.
- fixed an issue detecting multiline boilerplate markers.
- added an id 'pg-title-no-subtitle' to a span containing dc.title_no_subtitle

- fixed an EPUB3 problem affecting kobo reader when a book has more than 10 chapters - read order was being sorted as a string.
- for EPUB2, added a deprecation for `u` elements. fixes 210
- fixed multiline handling of MARKER_END in text boilerplate. fixes 209
- libgutenberg 0.10.20

- fixed reversion where adding display: initial over-wrote the addition of display:flex

- added `bgsound` to list of removed elements
- added 'initial` as an allowed value for the CSS properties `display`, `margin-*`, `font-*`, `text-*`, and `padding-*`. This has been part of W3C REC for over 2 years and is primarily useful for reproducing the behavior of the deprecated HTML4 align attribute in HTML5.
- Improved rendering of tables with respect to the deprecated `align` attribute.
- added handling for the deprecated `color` attribute on `hr` elements.
- updates pg urls in the backfile to https, updates links to html4 files to html5 files. fixed 194
- fixes 207 bug where gif files are converted to png, but the manifest gives them the wrong mediatype.
- libgutenberg 0.10.19

Nothing in this release should affect usage without Ebookconverter (e.g. Online Ebookmaker).
- Ebookmaker no longer needs a parsable source file to use metadata-only writers. So, RDF, and QRcodes, and posts to Mastodon and Facebook, can be made using only the database. This change should also save execution time, as parsing of the source files is no longer done for each of these outputs. RDF files can now be made for "books" that aren't books, such as data files, audio files, etc.
- ebook number on Logger is set even if parse fails.