support math functions

[fiji-flo]

• [ ] add1: Increment an integer by 1
• [ ] add: Sum an arbitrary number of integers
• [ ] sub: Subtract the second integer from the first
• [ ] div: Divide the first integer by the second
• [ ] mod: Module of first integer divided by second
• [ ] mul: Multiply integers
• [ ] max: Return the biggest of a series of one or more integers
• [ ] min: Return the smallest of a series of one or more integers
• [ ] biggest: DEPRECATED. Return the biggest of a series of one or more integers

[sstubbs]

I'm working on this now. Will do a pr. I need the until function but I might as well just try get these all done. Working on these http://masterminds.github.io/sprig/math.html and http://masterminds.github.io/sprig/integer_slice.html