Add support for looking up interface descriptions in SNMP and for forcing domain names in exported metrics.

This PR does two things:

1. It uses SNMP to lookup the IF-MIB::ifDescr for each interface if the SNMP_COMMUNITY environment variable is set, rather than just using the interface index in the metric lines.
2. It allows you to set a FORCE_DOMAIN environment variable to force the domain name for metrics. This is necessary because some devices have limitations over which network interfaces sFlow data can be exported, so it can be difficult to have reverse DNS map to the desired hostname.