Make bugz modify --fixed/--invalid configurable

`bugz modify --fixed` and `bugz modify --invalid` hardcode STATUS and RESOLUTION. Some instances of Bugzilla may rename them to better match their workflow.

It'd be nice if pybugz.d/*.conf could specify alternative names similar to `search_statuses` e.g.,

```
[FreeBSD]
resolved_status = Closed
invalid_status = Rejected
```
