return value of last expression from multi-line py"""..."""

Currenty,
```jl
py"""
....Python code
"""
```
returns nothing, because that is what [`Py_CompileString` does for `Py_file_input`](https://docs.python.org/2/c-api/veryhigh.html).

It would be nice to, instead, return the result of the last expression evaluated, similar to what IPython does when evaluating multi-line input. As discussed in #255, IPython does this by parsing the code and then evaluating one expression in the AST at a time, and in principle we could do the same thing.