EPUB issues

[BartMassey]

PR #39 added EPUB formatting to the book. This is great!

Sadly, a couple of things still need fixing.

• The EPUB stylesheet links in the headers are wrong. They currently look like

  <link rel="stylesheet" href="../stylesheet.css" />
  

  but should include a style attribute.

  <link rel="stylesheet" style="text/css" href="../stylesheet.css" />
  

  This seems mostly likely to be a deficiency in mdbook-epub: if so, it needs a separate issue and PR there.

• mdbook and mdbook-epub disagree about the permissibility of hyperlinks to .md files: mdbook fixes them, mdbook-epub does not. See for example the verify link at the end of Linux setup. Easiest fix is to change the book markdown to use .html links instead. This seems less good to me because future formats, but still quite livable.

I strongly recommend Sigil as an EPUB validation tool.