Add new flags to skdiff

Add new flags to skdiff

New flags are:
--nodiffs: don't write out image diffs or index.html, just generate report on stdout
--match: compare files whose filenames contain this substring; if unspecified, compare ALL files
         this flag may be repeated to add more matching substrings
--nomatch: regardless of --match, DO NOT compare files whose filenames contain this substring
           this flag may be repeated to add more forbidden substrings

Also implemented the --threshold flag, which was already documented but not implemented.
Review URL: https://codereview.appspot.com/6135045

git-svn-id: http://skia.googlecode.com/svn/trunk@3806 2bbb7eff-a529-9590-31e7-b0007b416f81