Added missing transformer in init

IPython/nbconvert/transformers/__init__.py

@@ -6,6 +6,7 b' from .extractfigure import ExtractFigureTransformer'
 from .revealhelp import RevealHelpTransformer
 from .latex import LatexTransformer
 from .sphinx import SphinxTransformer
+from .csshtmlheader import CSSHTMLHeaderTransformer
 
 # decorated function Transformers
 from .coalescestreams import coalesce_streams