Support rendering of `cftime` time axes

proplot doesn’t seem to know how to handle cftime too well, which is increasingly more common in xarray now and can handle various climate model calendars, making it pretty useful.

It looks like the auto-generated datetime axis fails and tries to fill in every single time step:

import numpy as np
import proplot as plot
import xarray as xr

datetime = np.arange('1990-01', '2000-01', dtype='datetime64[M]')
cftime = xr.cftime_range('1990-01', '2000-01', freq='M')
da_datetime = xr.DataArray(np.random.rand(len(datetime)), dims='time', coords=[datetime])
da_cftime = xr.DataArray(np.random.rand(len(cftime)), dims='time', coords=[cftime])

f, ax = plot.subplots()
%time ax.plot(da_datetime.time, da_datetime)

f, ax = plot.subplots()
%time ax.plot(da_cftime.time, da_cftime)

Timing comes out essentially equivalent but the latter hangs for 10-15 seconds, probably trying to render the image in the notebook.

[lukelbd]

I discovered the problem; will close this soon.

[lukelbd]

I decided to also release a new version (v0.5.0), which includes Fabio’s colormaps and this fix. Feel free to update.