feat(feh): deassociate with xcf, pdf, dvi, txt

- opening *.xcf (Gimp vectorial image) is mentioned in feh’s manpage
  but I have been unable to open any such file;

What is the problem in this case? According to the man page, it seems to use ImageMagick for .xcf when it is enabled by --conversion-timeout. There are many possibilities:

ImageMagick stopped supporting .xcf after the corresponding part of the man page of feh was written. In this case, we can drop .xcf from the list.
ImageMagick needs a special configuration to open .xcf. In this case, some users may set it up, so I'm not sure if we should simply drop it.
ImageMagick works, but feh cannot use the result of ImageMagick for some reason. In this case, maybe we should ask the upstream feh about whether this is intentional.

- opening *.ps, *.pdf, *.dvi (document files) works through ImageMagick,
  but only when the document contains a single page;
  + PDF and DVI are primarily used for possibly-multipage textual
    documents, PDF viewers are widely available and they are much better
    for this purpose than an image viewer that first converts documents
    to bitmaps; it’s unlikely that a user wants to use feh for these.

Is it? The images that will be embedded in PDFs are usually generated as single-page PDFs. I always create plots in academic papers as single-page PDFs. Although I'm not a user of feh, I'm not sure if we can easily conclude that no users use feh to preview images in the form of single-page PDFs.

scop / bash-completion

feat(feh): deassociate with xcf, pdf, dvi, txt #1224