Added svgo for svg optimization

With default options it varies from ~30% reduction for plantuml
generated, all the way up to 80 to 90 precent for inkscape based
images.

optimize_images/optimize_images.py

@@ -22,6 +22,7 @@ SHOW_OUTPUT = logger.getEffectiveLevel() <= logging.DEBUG
 # A list of file types with their respective commands
 COMMANDS = {
     # '.ext': ('command {flags} {filename', 'silent_flag', 'verbose_flag')
+    '.svg': ('svgo {flags} --input="{filename}" --output="{filename}"', '--quiet', ''),
     '.jpg': ('jpegtran {flags} -copy none -optimize -outfile "{filename}" "{filename}"', '', '-v'),
     '.png': ('optipng {flags} "{filename}"', '--quiet', ''),
 }