Options: Changed `writerSourceDir` to `writerSourceURL` (now a Maybe).

Previously we used to store the directory of the first input file,
even if it was local, and used this as a base directory for
finding images in ODT, EPUB, Docx, and PDF.

This has been confusing to many users.  It seems better to look for
images relative to the current working directory, even if the first
file argument is in another directory.

writerSourceURL is set to 'Just url' when the first command-line
argument is an absolute URL.  (So, relative links will be resolved
in relation to the first page.)  Otherwise, 'Nothing'.

The ODT, EPUB, Docx, and PDF writers have been modified accordingly.

Note that this change may break some existing workflows.  If you
have been assuming that relative links will be interpreted relative
to the directory of the first file argument, you'll need to
make that the current directory before running pandoc.

Closes #942.