Add a way to generate account numbers to the system

It would be nice if there was some way to auto-generate account numbers to the system. For example, a user could put in some pattern like:
```
%03d-(%03d)*
```

And it will put the top level account number, padded with zeros to make a 3-digit number, followed by a -, followed by the next account number, padded with zeros to make it 3 digits, concatenated by the next sub account, etc...

This seems to be a non-trivial problem, because regular expressions don't seem to be able to match repeated groups. So, we'd need to build a special regular expression for each account apparently.