Add a function to round to a specific decimal place

[silentmatt]

Basically the second half of #34 but separate from the round operator, which can't realistically support more than one operand.

[marwahaha]

Hi @silentmatt - it'd be great to have this. I'm ok building this but I noticed #34 already exists. Could I help out?

[silentmatt]

@marwahaha Sure, go for it. #34 is from before some major code reorganizations and changes so it can't be merged directly, but the implementation could be used as a starting point.

One thing to keep in mind is that the current round(x) "function" is actually an operator. The differences are subtle, and might go away eventually, but the bottom line is that for now, a two-argument version needs to be a separate function. I would probably go with roundTo, but I'm open to other suggestions.