Bug report

Bug summary

The xkcd context manager does not reset the config anymore

Code for reproduction

import matplotlib.pyplot as plt

with plt.xkcd():
    plt.figure()
    plt.plot([1, 2, 3])
    plt.savefig('xkcd.png', dpi=300)


plt.figure()
plt.plot([1, 2, 3])
plt.savefig('not_xkcd.png', dpi=300)

Actual outcome

Both figures are in xkcd style

Expected outcome
Second figure should be using default style

Matplotlib version

• Operating system:
• Matplotlib version: 2.1
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.6

using anaconda