Add Equal Earth projection

[QuLogic]

Rationale

For the reasons outlined in #1156.

Implications

We can now plot on the Equal Earth projection, though it only works with Proj 5.2.0+.

Note: This depends on #1180 for the false_easting/false_northing handling in the warped rectangular class.

[pelson]

Please could you update this PR now that #1180 is merged @QuLogic? On the whole, this looks great - I see no reason for this to not be in the release, but haven't yet completed a full review.

[QuLogic]

OK, looks like it's working, now that I figured out the Python 2 bugs.

[QuLogic]

Docs seem to downgrade to proj 4.9.3, so the projection is not being plotted in the list.

[pelson]

I've merged as the docs aren't a showstopper, and #1185 is moving us in the right direction.

Great addition. Thanks @QuLogic!