Undefined variable "${u}" referenced

```
apacheconfigtool` --interpolatevars example.ini
Failed to parse Apache config example.ini: Undefined variable "${u}" referenced
```
example.ini is the same [configuration file supplied with the documentation](https://github.com/etingof/apacheconfig/#interpolatevars).

It seems variable names are truncated to only their first character.  E.g. $user becomes $u.
