ENH: add selector value access for expressions

[jakevdp]

Addresses #1013

This PR adds the ability to access selector values within filter & condition expressions; for example:

import altair as alt
import pandas as pd
import numpy as np

df = pd.DataFrame({
    'xval': range(100),
    'yval': np.random.randn(100).cumsum()
})

slider = alt.binding_range(min=0, max=100, step=1, name='cutoff')
selector = alt.selection_single(fields=['cutoff'], bind=slider, init={'cutoff': 50})

alt.Chart(df).mark_point().encode(
    x='xval',
    y='yval',
    color=alt.condition(alt.datum.xval < selector.cutoff, alt.value('red'), alt.value('blue'))
).add_selection(
    selector
)

[jakevdp]

It would be useful to add an example and some documentation as well.