This resolves bug #2873142, Please add support for multiple embedded fonts

This resolves bug #2873142, Please add support for multiple embedded fonts

If you navigate to a checkout of DocBook-XSL and go to:
xsl/epub/bin/spec/files
You can now run the following command:

../../dbtoepub -f DejaVuSerif.otf -f DejaVuSerif-Italic.otf -c test.css
-s test_cust.xsl orm.book.001.xml

In dbtoepub, the following option can be used more than once:
-f, --font [OTF FILE] Embed OTF FILE in .epub.

The underlying stylesheet now accepts a comma-separated list of font file
names rather than just one as the RENAMED epub.embedded.fonts ('s' added).

The runnable EPUB spec now includes:
- should be valid .epub after including more than one embedded font