Closes #207.

pyplot "attaches" a figure to a backend on calling pyplot.figure. Previously we
implemented a hack that worked around this limitation in case the user did not
import pyplot. However this hack is fragile and breaks new things with newer
matplotlib versions. Now we detect whether the figure is intended for pyplot
and import pyplot directly if it is.

This change also allows us to simplify the interface and the implementation of
output_fig, but this is permitted because it is an internal function.