support more filter functions

[yokolet]

Currently, it supports very simple functions only. (ex. greater than, less than)

I think we'd better support user defined clojure functions, first, then ruby method.

For example, accepts string form of clojure function that needs one argument. The argument is given as an argument, not in the part of function. Then, creates var type using clojure's read-string function, and so on.

Perhaps, it's better to have arity check.