[low priority] Some feature requests

I'd like to request 3 low priority features:
1. output as one pdf file instead of multiple png
2. include _all_ pdf pages are included (not just ones containing figures) so that I can check for missed figures/tables/captions
3. allow prefix to be specified once when using -a and -j options together

Feature 1 can be achieved with imagemagick easily:

```
# cd to folder containing pdf files to parse and
PREFIX=/path/to/output/fileprefix
for filepath in *.pdf; do
echo processing $filepath ...
file="${filepath%.*}"
pdffigures -a "$PREFIX/$file" -j "$PREFIX/$file" $file.pdf
#imagemagick convert
convert `ls $PREFIX/$file*.png` "$PREFIX/$file.pdf"
rm `ls $PREFIX/$file*.png`
done;
```