Add function support to the query language

For 99% of cases I don't think they're really necessary, but it might be nice to have some functions such as `length(str)` in the language, for example if you wanted to query customers who provided abnormally large URLs:

```
length(url) > 256
```

Some useful functions might be:

- string functions for trimming, replacing etc
- math functions
- functions for extracting date/time components?
- functions to check formats (is a valid ip address etc?)

Let me know if there are any you think would be useful.