Keyword arguments to Context.execute as globals

The current Context.execute(script) is a bit limited in terms of user friendliness of passing Python values to the JS interpreter. The current approach is to define a set of global values using Context.add_global().

I should update Context.execute to apply any keyword arguments as globals in the JavaScript interpreter. Something like:

assert cx.execute("var f = 2 * bar + omega; f;", bar=10, omega=13) == 33