Dry-run mode

Hi @grke,

I am currently implementing a *configuration panel* for burp in burp-ui.
I thought it would be nice to have a *dry-mode* in burp in order to test the syntax of the configuration file, so here is a patch to have a basic *dry-run* mode.

It could be improved by dumping all encountered errors at once in the future.

During my tests I also noticed the burp configuration file parser might not be very accurate. For instance, it seems that I can setup something like:

```
syslog = 3
```