Support -a flag in snmp-interface check

[allanlewis]

Is your feature request related to a problem? Please describe.

I'd like to be able to monitor interfaces without caring if the link is up. The snmp-interface check supports this via the following option:

-a, --admin
   Use administrative status instead of operational

Describe the solution you'd like

Add support for the -a/--admin option to the snmp-interface plugin.

Describe alternatives you've considered

I'd have to implement some logic to determine which interfaces' links should be up.

Additional context

It would be good to support all options provided by the latest version of this plugin; there are several others that currently aren't supported.