Provide access to GMT native colormaps

As far as I can tell, it is currently not possible to make use of the native GMT colormaps outside of the pygmt routines. There are lot of reasons of why you might want to use these elsewhere. For example, you might want to plot colored x-y data in a matplotlib image and use the same colormap in an accompanying pygmt map.

Some of these colormaps (such as Fabio Crameri's) can be downloaded and installed separately. Nevertheless, it would be much more convenient if these colormaps were all included explicitly in pygmt (such as in a module cmap, for example).