Add support for if-then-else operator to expressions

[zdila]

I am missing ternary if-then-else expression operator (in C: cond ? trueExp : falseExp). I started to implement it but now I am struggling with it. Please see mapnik/mapnik-support#133 (comment). I will appceriate any hints to finish the implementation myself but also if somebody else would take it over.

[zdila]

For numeric values it can be emulated with cond * truthyValue + not(cond) * falsyValue. Not sure about strings.

Alternative implementation could use function: if(cond, truthyValue, falsyValue).