Facilitate command line usage

Created by: Ede123

• Allow the common way to specify input/output files as positional arguments, e.g.
  scour input.svg output.svg.
• Make --help output much clearer by using option groups:

scour 0.33
Copyright Jeff Schiller, Louis Simard, 2010
Usage: scour.py [INPUT.SVG [OUTPUT.SVG]] [OPTIONS]

If the input/output files are not specified, stdin/stdout are used. If the
input/output files are specified with a svgz extension, then compressed SVG is
assumed.

Options:
  --version                      show program's version number and exit
  -h, --help                     show this help message and exit
  -q, --quiet                    suppress non-error output
  -v, --verbose                  verbose output (optimization statistics,
                                 etc.)
  -i INPUT.SVG                   alternative way to specify input filename
  -o OUTPUT.SVG                  alternative way to specify output filename

  Optimization:
    -p NUM, --set-precision=NUM  set number of significant digits (default: 5)
    --disable-simplify-colors    won't convert all colors to #RRGGBB format
    --disable-style-to-xml       won't convert styles into XML attributes
    --disable-group-collapsing   won't collapse <g> elements
    --create-groups              create <g> elements for runs of elements with
                                 identical attributes
    --keep-editor-data           won't remove Inkscape, Sodipodi, Adobe
                                 Illustrator or Sketch elements and attributes
    --keep-unreferenced-defs     won't remove elements within the defs
                                 container that are unreferenced
    --renderer-workaround        work around various renderer bugs (currently
                                 only librsvg) (default)
    --no-renderer-workaround     do not work around various renderer bugs
                                 (currently only librsvg)

  SVG document:
    --strip-xml-prolog           won't output the XML prolog (<?xml ?>)
    --remove-metadata            remove <metadata> elements (which may contain
                                 license/author information etc.)
    --enable-comment-stripping   remove all comments (<!-- -->)
    --disable-embed-rasters      won't embed rasters as base64-encoded data
    --enable-viewboxing          changes document width/height to 100%/100%
                                 and creates viewbox coordinates

  Output formatting:
    --indent=TYPE                indentation of the output: none, space, tab
                                 (default: space)
    --nindent=NUM                depth of the indentation, i.e. number of
                                 spaces/tabs: (default: 1)
    --no-line-breaks             do not create line breaks in output(also
                                 disables indentation; might be overriden by
                                 xml:space="preserve")
    --strip-xml-space            strip the xml:space="preserve" attribute from
                                 the root SVG element

  ID attributes:
    --enable-id-stripping        remove all unreferenced IDs
    --shorten-ids                shorten all IDs to the least number of
                                 letters possible
    --shorten-ids-prefix=PREFIX  add custom prefix to shortened IDs
    --protect-ids-noninkscape    don't remove IDs not ending with a digit
    --protect-ids-list=LIST      don't remove IDs given in this comma-
                                 separated list
    --protect-ids-prefix=PREFIX  don't remove IDs starting with the given
                                 prefix