added iplot_mpl() and plotly_takeover()

`iplot_mpl()` allows offline matplotlib figure conversion
for ipython notebook inline mode.

`plotly_takeover()`  is a compound command that allows an entire
notebook session that was written using matplotlib to use offline
plotly without any modification.

the later idea was taken from
https://github.com/jakevdp/mpld3/blob/202792bcddbfe957ade08a5085691ab176a4563d/mpld3/_display.py#L363

Author: arsenovic

plotly/offline/__init__.py

@@ -6,5 +6,7 @@
 from . offline import (
     download_plotlyjs,
     init_notebook_mode,
-    iplot
+    iplot,
+    iplot_mpl,
+    plotly_takeover,
 )

plotly/offline/offline.py

@@ -16,6 +16,19 @@
 from plotly.exceptions import PlotlyError
 
 
+try:
+    import IPython
+    _ipython_imported = True
+except ImportError:
+    _ipython_imported = False
+
+try:
+    import matplotlib
+    _matplotlib_imported = True
+except ImportError:
+    _matplotlib_imported = False
+
+
 __PLOTLY_OFFLINE_INITIALIZED = False
 
 
@@ -171,6 +184,34 @@ def iplot(figure_or_data, show_link=True, link_text='Export to plot.ly',
                            height=height, width=width)))
 
 
+def iplot_mpl(mpl_fig,mpl_to_plotly_kw={},iplot_kw={}):
+    '''
+    Convert a matplotlib figure to plotly dictionary  plot inside an 
+    IPython notebook without connecting to an external server.
+    '''
+    plotly_plot = tools.mpl_to_plotly(mpl_fig,**mpl_to_plotly_kw)
+    return iplot(plotly_plot,**iplot_kw)
+
+def plotly_takeover(**kwargs):
+    '''
+    Enable the automatic display of figures in the IPython Notebook.
+    This function should be used with the inline Matplotlib backend
+    that ships with IPython that can be enabled with `%pylab inline`
+    or `%matplotlib inline`. This works by adding an HTML formatter
+    for Figure objects; the existing SVG/PNG formatters will remain
+    enabled.
+    
+    (idea taken from `mpld3._display.enable_notebook`)
+    '''
+    if __PLOTLY_OFFLINE_INITIALIZED != True:
+        init_notebook_mode()
+    ip = IPython.core.getipython.get_ipython()
+    formatter = ip.display_formatter.formatters['text/html']
+    formatter.for_type(matplotlib.figure.Figure,
+                       lambda fig, kwds=kwargs: iplot_mpl(fig, **kwds))
+
+    
+
 def plot():
     """ Configured to work with localhost Plotly graph viewer
     """