Support layering aes() multiple times

[jdanbrown]

Bug: using aes() multiple times loses all but the last one

from lets_plot import *
from plotnine.data import mpg

LetsPlot.setup_html()

(
    ggplot(mpg)
    + aes(x='hwy')
    + aes(y='cty')
    + geom_point()
)

Using exactly one aes() does the right thing

from lets_plot import *
from plotnine.data import mpg

LetsPlot.setup_html()

(
    ggplot(mpg)
    + aes(x='hwy', y='cty')
    + geom_point()
)

Expected behavior (from plotnine)

from plotnine import *
from plotnine.data import mpg

LetsPlot.setup_html()

(
    ggplot(mpg)
    + aes(x='hwy')
    + aes(y='cty')
    + geom_point()
)

from plotnine import *
from plotnine.data import mpg

LetsPlot.setup_html()

(
    ggplot(mpg)
    + aes(x='hwy', y='cty')
    + geom_point()
)