Add Natural Earth plotting tools

[juseg]

Add Natural Earth functions from cartowik. This is essentially a thin wrapper around cartopy, adding a little convenience, and a lot of speed in some cases.

• [x] Add shapefile partial plotting and docstring.
• [x] Add Natural Earth functions and docstrings.
• [x] Use annotated scatter for custom cities maps.
• [x] Update examples, cartopy section in docs.
• [x] New doc page on adding map elements.