expose some transformers at higher level

nbconvert/transformers/__init__.py

@@ -0,0 +1,10 b''
+
+# Class base Transformers
+from activatable import ActivatableTransformer
+from base import ConfigurableTransformer
+from extractfigure import ExtractFigureTransformer
+from latex import LatexTransformer
+from sphinx import SphinxTransformer
+
+# decorated function Transformers
+from coalescestreams import coalesce_streams