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.
Please uppercase "OID" so it's consistent with the writing in other error messages.