title: How to add subplot title to 3D subplot in plotly
body: <p>I am plotting a images with 4 subplots, e.g. using the following code.
  How can I add title to each subplot?</p>
  
  <pre><code>import plotly.offline as py
  import plotly.graph_objs as go
  from plotly import tools
  
  import numpy as np
  
  x = np.linspace(-5, 80, 10)
  y = np.linspace(-5, 60, 10)
  xGrid, yGrid = np.meshgrid(y, x)
  z = xGrid ** 3 + yGrid ** 3
  
  scene = dict(
      xaxis=dict(
          gridcolor='rgb(255, 255, 255)',
          zerolinecolor='rgb(255, 255, 255)',
          showbackground=True,
          backgroundcolor='rgb(230, 230,230)'
      ),
      yaxis=dict(
          gridcolor='rgb(255, 255, 255)',
          zerolinecolor='rgb(255, 255, 255)',
          showbackground=True,
          backgroundcolor='rgb(230, 230,230)'
      ),
      zaxis=dict(
          gridcolor='rgb(255, 255, 255)',
          zerolinecolor='rgb(255, 255, 255)',
          showbackground=True,
          backgroundcolor='rgb(230, 230,230)'
      )
  )
  
  fig = tools.make_subplots(rows=2, cols=2,
                          specs=[[{'is_3d': True}, {'is_3d': True}],
                                  [{'is_3d': True}, {'is_3d': True}]])
  
  # adding surfaces to subplots.
  fig.append_trace(dict(type='surface', x=x, y=y, z=z, colorscale='Viridis',
                      scene='scene1', showscale=False), 1, 1)
  fig.append_trace(dict(type='surface', x=x, y=y, z=z, colorscale='RdBu',
                      scene='scene2', showscale=False), 1, 2)
  fig.append_trace(dict(type='surface', x=x, y=y, z=z, colorscale='YlOrRd',
                      scene='scene3', showscale=False), 2, 1)
  fig.append_trace(dict(type='surface', x=x, y=y, z=z, colorscale='YlGnBu',
                      scene='scene4', showscale=False), 2, 2)
  
  fig['layout'].update(title='subplots with different colorscales',
                      height=800, width=800)
  fig['layout']['scene1'].update(scene)
  fig['layout']['scene2'].update(scene)
  fig['layout']['scene3'].update(scene)
  fig['layout']['scene4'].update(scene)
  
  py.plot(fig, filename='multiple_plots.html')
  </code></pre>
  
  <p>I tried to replace</p>
  
  <pre><code>fig = tools.make_subplots(rows=2, cols=2, specs=[[{'is_3d': True}, {'is_3d': 
  True}], [{'is_3d': True}, {'is_3d': True}]])
  </code></pre>
  
  <p>with the following </p>
  
  <pre><code>fig = tools.make_subplots(rows=2, cols=2, subplot_titles= ('Plot1','Plot2','Plot3','Plot4'),specs=[[{'is_3d': True}, {'is_3d': True}],[{'is_3d': True}, {'is_3d': True}]])
  </code></pre>
  
  <p>It gives me the following error</p>
  
  <hr>
  
  <p>IndexError                                Traceback (most recent call last)
   in ()
       33 fig = tools.make_subplots(rows=2, cols=2,subplot_titles= ('p1','p2','p3','p4'),
       34                           specs=[[{'is_3d': True}, {'is_3d': True}],
  ---> 35                                  [{'is_3d': True}, {'is_3d': True}]])
       36 
       37 # adding surfaces to subplots.</p>
  
  <p>~\AppData\Local\Continuum\anaconda3\lib\site-packages\plotly\tools.py in make_subplots(rows, cols, shared_xaxes, shared_yaxes, start_cell, print_grid, **kwargs)
     1373             pass
     1374         else:
  -> 1375             plot_titles.append({'y': subtitle_pos_y[index],
     1376                                 'xref': 'paper',
     1377                                 'x': subtitle_pos_x[index],</p>
  
  <p>IndexError: list index out of range</p>
  
  <p>How can I solve this?
  Thanks</p>