Support enabling/disabling VegaFusion renderers as context managers

[jonmmease]

Closes #200

Adds support for enabling and disabling VegaFusion renderers/data transformers in context managers.

The following now work as you would expect:

from IPython.display import display
with vf.enable_mime():
    display(chart)

with vf.disable():
    display(chart)

with vf.enable_widget():
    display(chart)

cc @mattijn

[mattijn]

Much promising 👍