Fix ValueError from repeated import ehtplot.color

[endolith]

If a script importing this module is run more than once, it produces

ValueError: A colormap named "afmhot_10u" is already registered.

Now it overwrites the existing colormaps instead, and fixes #5.

[endolith]

Actually this seemed to work fine when I tested it, but now that I'm actually using it to produces a raft of warnings:

… 
…\core.py:45: UserWarning: Overwriting the cmap 'YlOrRd_ur' that was already in the registry.
  register_cmap(name=name + ("_r" if unmodified(name) else "r"),
…\core.py:42: UserWarning: Overwriting the cmap 'YlOrRd_us' that was already in the registry.
  register_cmap(name=name, cmap=cmap, override_builtin=True)
…\core.py:45: UserWarning: Overwriting the cmap 'YlOrRd_usr' that was already in the registry.
  register_cmap(name=name + ("_r" if unmodified(name) else "r"),

So maybe that's not the best approach.