Add ScatterGeo trace support

[andrei-ng]

Fixes #300

[andrei-ng]

@G0rocks , I have squashed your commits and added the Geo you pushed in the latest commit with some modifications into the Layout struct. Now I can generate the shape as in the example you posted in #300 , except the traces as I didn't spend time into writing the for loop to replicate this example exactly.

The example is in examples/maps and you can generate the same as in the screenshot when you run cargo run and open the generated output/scatter_geo.html

The Geo Layout we have added in this PR does not have all the fields listed in the API https://plotly.com/javascript/reference/layout/geo/ , but I would rather merge this in and add the rest per a need basis.

Try it out and let me know if it is good enough to merge or you need more to have a functional ScatterGeo.