Add '-dcrop' option to ps and svg backends

Add '-dcrop' option to ps and svg backends

This change allows the output of scores in the format provided by the
'-dpreview' option but including all systems, not just the first.

This would allow for easier SVG use in HTML, without the need for
cropping snippets. Further, SVG is an HTML standard, and its vector
nature makes its use unparalleled for the web. This change would allow
SVG use in 'lilypond-book' in the future.