Make the use of MIB files completely optional

My particular use case required publishing some SNMP values by directly specifying the Oids, rather than names to extract from MIB files.

This is the change I had to implement in order to support such scenario. If you think it is useful for your users, please feel free to pull.

Thanks for your work on this library, by the way, It has made my project viable within the time constraints I had.
Initially I tried to do that, but since the system I was running this on did not have any mib files at all, the call to `netsnmp_init_mib` below was raising an exception. And I could not bypass that initialisation due to an empty MIB files list because somebody might just not want to load any custom MIB file, but still want to use the system provided ones.