add "yamlmibdir" option to snmptrap input.

This lets you load your own MIBs into the snmptrap input (replacing the built-ins in ruby-snmp).

Due to the way ruby-snmp works, it's not possible to augment the built-ins; has to be one or the other. [ at time of writing, the built-ins don't work anyway, so.... :) ]

This could probably do with some extra documentation on how to convert .mib files into the format ruby-snmp uses. If you tell me where that should live, I'll write something up.