Provide API for registering other functions.

[noahlz]

We are interested in using your library but would like to extend it to other types, for example Saddle Vecs.

For example, be able to define operators that are Map[String, Vec[Double] => Vec[Double]]

As it stands now, we have to fork your library, and rewrite object FunctionRegister to have two different function registers: OperatorDouble and OperatorVector

(Saddle is a Scala library for numeric operations not unlike Pandas - http://saddle.github.io/)

[noahlz]

I've asked my colleague @fshao816 to submit a PR with a proposed change.

[facaiy]

You are welcome, @noahlz . Thanks very much!