[WiP] Better support for probing MIBs with objects in different OID groups

[SpComb]

Fixes #15

A MIB has a primary OID used for resolving/formatting IDs like TEST-MIB.1.2.1, but it also has a set of OIDs to cover all objects/tables within the MIB.

This OIDSet is used for probing the MIB, and the presence of anything under any oid in the set signifies support for that MIB.

TODO

• Generate extra OIDs for MIBs in mib-import.py

  Loading of MIBs is currently broken:

    2018/05/12 16:21:57 Failed to load MIB P-BRIDGE-MIB objects from /home/terom/go/src/github.com/qmsk/snmpbot-mibs/P-BRIDGE-MIB.json: Unknown OID .1.3.6.1.2.1.17.4.5.1.1 for Object dot1dTpHCPortInFrames: outside of MIB OIDs {.1.3.6.1.2.1.17.6}