title: Output results of `mix hex.docs offline` as EPUB instead of HTML
body: <p>I keep offline copies of the documentation for Hex packages via <code>mix hex.docs offline PACKAGE_NAME</code>. This creates an HTML directory in <code>~/.hex/docs/hexpm/PACKAGE_NAME/</code>.</p>
  
  <p>I would like to do the same thing, but with the output as EPUB instead of HTML, and <strong>ideally, in one step</strong>.</p>
  
  <p>How can I download offline EPUBs for documentation of packages hosted on Hex.pm?</p>