Add smarter parameter processing to setx

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Currently setx requires something like:

setx(z.out, age = sum(turnout$age))

where turnout is the data.frame and age is a column within turnout

Let's support syntax that's like this: setx(z.out, age = sum(age))

That is, intelligently process parameters in the function. This can be simply done with the with function

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Making this a requirement for beta, since it immensely simplifies using Zelig, and makes function calls make semantic sense

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That syntax now works. I think it's a huge intuitive improvement