dm: support printing specific page of mskp to SVG

Currently with dm, it's possible to convert an .mskp to a multi-page PDF as
follows:

    out/Release/dm --src mskp --mskps /tmp/filename.mskp -w /tmp \
    --config pdf --verbose

The SVG equivalent partially works, although only outputs the first page:

    out/Release/dm --src mskp --mskps /tmp/filename.mskp -w /tmp \
    --config svg --verbose

This CL adds support for passing extended options to SVG.  Specifically, the
'page' option , which now determines which page of the source mskp gets
converted to the SVG output.  The new syntax is as follows:

    out/Release/dm --src mskp --mskps /tmp/filename.mskp -w /tmp \
    --config svg[page=2] --verbose

The `[key=value]` syntax is the same extended options syntax currently used by
dm with --config gpu, e.g. `gpu[api=gl,color=8888]`.

BUG=skia:7601

Change-Id: I3523d79b1cdbbba9e80fd46501331877091bdead
Reviewed-on: https://skia-review.googlesource.com/105404
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>

0 files changed, 0 insertions, 0 deletions