Add --nocache option to force a bugzilla query.

When a bugzilla entry is changed the script might not pick it up due to caching. You can already force requerying by re-compiling the script (or e.g. running via --force with RDMD), but this should be a runtime option, not a compile-time one. 

I've named it `nocache` rather than `force` since the latter could be confusing when called via RDMD, e.g.:

```
rdmd --force changed.d --force --start=2013-11-05
```