Add convenience function to look up a single value in a `DataFrame` about dataframesmeta.jl

[pdeffebach]

That particular syntax isn't actually possible, since @lookup would only apply to the expression inside the brackets.

Moreover, I would definitely want to avoid that kind of exotic syntax. DataFrames.jl doesn't have any data.table style indexing, and I think the initial proposed syntax is good.

[nathanrboyer]

Since this is a meta package, it may not need to look like a function call at all. The syntax could be something more exotic?

zvalue = df[@lookup, :z]  :x > 1, :y == "A"