Ebookmaker

- tweak boilerplate to align with PG practice
- use `*** START OF THE` instead of `*** START OF THIS`. PG shifted from `THIS` to `THE` around 64000.

- changed caching strategy after analysis showed large cost for changes made in 0.12.12

- handle pre-html5 type attribute of `ul` properly
- continue processing job queue after a parse failure

- fix bug in removal of legacy elements
- add back enclose_text removed in 0.12.14; it wasn't redundant
- when an img is removed, replace it with alt text. fixes 123
- It seems we have to use the EPUB2 method to declare the cover image for EPUB3 files.

- fixed bug exposed by implementation of downloaded pubinfo in libgutenberg
- updated libgutenberg to 0.10.5 to fix an older bug revealed by rendering bugfix

- add tests for conversion from txt source
- boilerplate marking was duplicating an id. The code was incoherent. Cleaned this up.
- don't turn top-level html comments into non-comments. in BS4, Comment is a subclass of NavigableString! Thanks G4OEU!
- removed redundant enclose_text()
- `--make=kindle` and `--make=all` no longer make `kindle.noimages`
- stop warning about external links that start with 'https://www.gutenberg.org/'
- More helpful message when file not found