Add watermark plugin

tests/sample/sigal.conf.py

@@ -10,10 +10,12 @@ keep_orig = True
 links = [('Example link', 'http://example.org'),
          ('Another link', 'http://example.org')]
 
-plugins = ['sigal.plugins.adjust', 'sigal.plugins.copyright']
+plugins = ['sigal.plugins.adjust', 'sigal.plugins.copyright', 'sigal.plugins.watermark']
 copyright = u"© An example copyright message"
 adjust_options = {'color': 0.0, 'brightness': 1.0,
                   'contrast': 1.0, 'sharpness': 0.0}
-
+watermark = "watermark.png"
+watermark_position = "tile"
+watermark_opacity = 0.3
 # theme = 'galleria'
 # thumb_size = (280, 210)