an attempt to fix register_cmap import in plotting

[golobor]

addresses #520

[golobor]

@Phlya , i've requested your review, b/c you seem to have fixed #520 before (dc1fdd9) , but the fix didn't work for me. Did your fix work for you?

I think the reason your version doesn't work for me is b/c matplotlib.colormaps is not a module, but a variable.
Here is where it gets imported into the matplotlib namespace from mpl.cm:
https://github.com/matplotlib/matplotlib/blob/857e7d8aacd52d552e2e617ce23da30166419dcd/lib/matplotlib/__init__.py#L1515
And here is where it gets defined:
https://github.com/matplotlib/matplotlib/blob/857e7d8aacd52d552e2e617ce23da30166419dcd/lib/matplotlib/cm.py#L238

In any case, the new version should be agnostic to whether mpl.colormaps is a module or a variable. Would you green-light it?

[Phlya]

Did they also swap the argument order? WTF. OK, looks good to me, thanks for fixing it again!

[golobor]

ah, good catch re: argument order. I will use kwargs to make sure that it's backward compatible.