Add Recursive option in command line interface

In my case, I have more than 5000 SVG files stored in a directory tree and recursion would ease my work.

This option improve the command line interface with file's selection in directories recursively.

  -r, --recursive
      Select subdirectories recursively

Example :

                Remove all unused items in all SVG files in current directory and subdirectories
                inkscape -r *.svg --vacuum-defs