text: Title: datetime locator doesn't work with cftime
  Body: #### Code sample, a copy-pastable example if possible
  
  ```python
  import proplot as plot
  import numpy as np
  import xarray as xr
  
  times = xr.cftime_range('1990', '2000', freq='M')
  data = xr.DataArray(np.random.rand(len(times)),
      dims=['time'],
      coords=[times])
  
  f, ax = plot.subplots(aspect=2)
  
  ax.plot(data.time, data)
  
  # ax.format(xlocator='year')
  # ax.format(xformatter='concise')
  ```
  
  #### Actual result vs. expected result
  
  The x datetime axis appears by default (with the comments included above), but when using `ax.format(xlocator='year')` or any locator, the xticks and labeling completely disappear. 
  
  When using any formatter (e.g. `xformatter='concise')` it breaks with the following error:
  
  ```python-traceback
  ValueError: Cannot convert -3652 to a date.  This often happens if non-datetime values are passed to an axis that expects datetime objects.
  ```