ENH: make all chart objects accept data as a first argument

[jakevdp]

I think this would make things a bit more uniform. For example, you could create a layered chart like this:

chart = alt.LayerChart(data).encode(x='x', y='y')
chart += alt.Chart().mark_point()
chart += alt.Chart().mark_line()

and similar for HConcatChart, VConcatChart, etc.

Currently, the first argument to these classes is the list of child charts.