Context support

[cvanelteren]

Allows for using a temporary style on a plot. This builds on-wards from #459

import proplot as plt
import numpy as np


fig, ax = plt.subplots(ncols=3)
ax.imshow(np.random.rand(10, 10))
ax.format(xlabel="x")

with plt.context("poster", after_reset=True):
    fig, ax = plt.subplots()
    ax.imshow(np.random.rand(10, 10))
    ax.format(xlabel="x")

fig, ax = plt.subplots()
ax.imshow(np.random.rand(10, 10))
ax.format(xlabel="x")
plt.show(block=1)

[cvanelteren]

Just saw that this is already implemented: