Configurable floating point precision

The URL-friendliness goes away as soon as floating point numbers are used at all.  It would be nice to be be able specify that we only need 3 digits (or whatever) after the decimal point, rather than the full 15 we get now.

```
2.700000047683716,15.149999618530273 < not URL friendly   
2.700,15.150                         < much better
```