Arrange a mechanism whereby each backend can be passed a filename

Arrange a mechanism whereby each backend can be passed a filename
from its command-line option (`--text=foo.txt') and automatically
convert it into one or more notional \cfg directives. In the HTML
case this mechanism enables single-file mode as well as setting the
filename.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4018 cda61777-01e9-0310-a592-d414129be87e