[enhancement] show a few more contributor roles

[Wandmalfarbe]

Currently there are three contributor roles extracted from the ebook metadata.

1. editors
2. illustrators
3. translators

It would be nice to have a few more such as cover designer, printer, book designer, compositor or reviewer. A complete list of the available roles can be seen here http://www.loc.gov/marc/relators/relaterm.html.

Example for cover designer:

<dc:contributor opf:file-as="Glazer-Naude, Ludvik" opf:role="cov">Ludvik Glazer-Naude</dc:contributor>

Example for printer:

<dc:contributor opf:role="prt">Ebner &amp; Spiegel</dc:contributor>

Example for compositor:

<dc:contributor opf:file-as="Reischmann, Barbara" opf:role="cmt">Barbara Reischmann</dc:contributor>