ENH: encodings with multiple fields (e.g. tooltip)

[jakevdp]

We need to update the encode() method so that it will handle multiple encoding fields in cases where it is supported.

For example, in the most recent vega-lite release, it is possible to pass multiple fields to the tooltip encoding:

from vega_datasets import data
cars = data.cars()

chart = alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin'
)

chart.encoding.tooltip = [{'field': 'Name', 'type': 'nominal'},
                          {'field': 'Origin', 'type': 'nominal'}]

chart

Altair should make this available via a more simplified API, such as

from vega_datasets import data
cars = data.cars()

chart = alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
    tooltip=['Name', 'Origin']
)

[kanitw]

detail also support multiple fields (since v1).