ENH: allow for multiple stc views via mlab figures

I've added a figure kwarg that allows to pass a custom self-created figure
to plot_source_estimates.

You now can say things like:

```Python
import mne
from mayavi import mlab

stc= mne.read_source_estimate('my-sourc-lh.stc')

fig1 = mlab.figure('view1')
brain1 = stc.plot(figure=fig1)
brain1.set_time(0.1)

fig2 = mlab.figure('view2')
brain2 = stc.plot(figure=fig2)
brain2.set_time(0.15)

```