ENH: add datatype geojson

[mattijn]

This PR adds the :G option for geojson data type. As was found out in #1711 this was missing in Altair.

With this PR the following example (part of this PR) will work:

import altair as alt
from vega_datasets import data

states = alt.topo_feature(data.us_10m.url, 'states')
source = data.income.url

alt.Chart(source).mark_geoshape().encode(
    shape='geo:G',
    color='pct:Q',
    tooltip=['name:N', 'pct:Q'],
    facet='group:N'
).transform_lookup(
    lookup='id',
    from_=alt.LookupData(data=states, key='id'),
    as_='geo'
).properties(
    width=300,
    height=175,
    columns=2
).project(
    type='albersUsa'
)

[jakevdp]

Looks great, thanks!

[mattijn]

Grmpf. The PR of #1707 was in my local master when I made a new branch for this current PR. To make the current PR clean I removed these commits.

But now this last commit is also applied on the master of Altair.
So my last commit on this PR removed #1707 again..:(

Any advice how to solve?