Add support for complex arithmetic

[mabotkin]

I'm currently looking for a library to parse an expression in javascript and be able to evaluate it with complex variables as input. I saw that complex.js was already included so I was wondering if there were plans to have support for complex variables in the future. I could work on it if necessary.

[superMDguy]

I was interested in a similar thing. It seems like complex numbers are built in, but it doesn't recognize them with the algebra.parse() function.

[nicolewhite]

Please feel free to work on this @mabotkin. I did explore complex numbers at some point (hence the stuff in complex.js) but did not get around to incorporating it.