Add an expression transformer

[yaeldekel]

The expression transformer takes the expression in the form of text using syntax of a simple expression language, and performs the operation defined in the expression on the input columns in each row of the data. The transformer supports having a vector input column, in which case it applies the expression to each slot of the vector independently. The expression language is extendable to user defined operations.

[justinormont]

I certainly would appreciate this addition. Mainly for its simplicity.