Support SVG images

[grst]

Test case:

%config InlineBackend.figure_format = 'svg'

n = 256
X = np.linspace(-np.pi,np.pi,n,endpoint=True)
Y = np.sin(2*X)

fig, ax = plt.subplots( nrows=1, ncols=1 )
ax.plot (X, Y+1, color='green', alpha=1.00)
ax.plot (X, Y-1, color='green', alpha=1.00)
plt.show()