added ability to drop units on toString

I have been working js quantities into an application I am developing and ran into wanting being able to use toString to return only the scalar and not the number. My specific use case is to convert it to a different unit and assign a precision (in my case of 1 position). I don't want the unit text though (using degree symbol).

I went ahead and added another parameter for the toString number which if left out should default to failing the false check. I have not written any tests, but would be open to writing some if it seems that my proposed addition would get merged in.

Hopefully I am going about this right, haven't done very many pull requests so far.