Allow overwriting of filters

[xSmurf]

It would be good if default filters living in say /usr/share/roflcoptor/filters could overwritten from /etc/roflcoptor/filters

[david415]

does this mean that you want roflcoptor to use filters from two directories? right now we specify the filter directory in the configuration file.

the downside to having a secondary and a primary override directory for filter policies is that it's not as clear which filters are being applied.