fix aes(): allow mixed positional and keyword arguments

aes() used to silently ignore additional keyword argument if some
positional arguments were used.

aes("x", "y", color="color") is now valid code and equivalent to
aes("x", "y", "color") or aes(x="x", y="y", color="color")
Merge pull request #65 from astanin/master

fix aes(): allow mixed positional and keyword arguments