zathura-pdf-mupdf (zathura support for mupdf backend)

The zathura-pdf-mupdf plugin adds PDF support to zathura by using the
mupdf rendering library (replacing the poppler-based PDF support from
the zathura package).

To switch between the mupdf and poppler backends without reinstalling
packages, adjust the symlink at /usr/lib(64)/zathura/pdf.so. It
should point to either pdf-mupdf/pdf.so or pdf-poppler/pdf.so. You can
also reinstall zathura to switch to the poppler backend, or reinstall
zathura-pdf-mupdf to switch to the mupdf backend (whichever was installed
last "wins").
