Enable 'Equal Earth' projection

[Equal Earth](https://en.wikipedia.org/wiki/Equal_Earth_projection) is an aesthetically pleasing, fairly recent equal-area pseudocylindrical projection, which has has been adopted by the NASA Goddard Institute for Space Studies.

Author: apparebit

src/plots/geo/constants.js

@@ -34,6 +34,7 @@ exports.projNames = {
     eckert5: 'eckert5',
     eckert6: 'eckert6',
     eisenlohr: 'eisenlohr',
+    'equal earth': 'equalEarth',
     equirectangular: 'equirectangular',
     fahey: 'fahey',
     'foucaut sinusoidal': 'foucautSinusoidal',