Log Message:
tex-tex4ebook{,-doc}: update to 0.4

New features:
- added new command line option: --build-dir. It moves auxiliary and temporary
  files to a specified directory.
- added `\epubpage` command - add a page break to the document.

Bug fixes:
- don't set PNG as a picture format explicitly to support SVG output.
- fixed issues with the detection of the zip command on Windows.
- save author and date globally.
- remove spurious section numbers from TOC in the Epub 3 output.