Adds CLI reference for the previously undocumented vlan <id> mgmt
command, including the disable case (vlan 0 mgmt), default state,
and a lockout warning.
Fixes#235
Added new `vlan show` command to dump current VLAN settings.
Supports printing PVID & ingress filtering per port.
Added new `ingress [p]<mode>` command to setup ingress filtering.
Added wrappers for enabling/disabling vlan filtering. Currently
not configurable, but state in console is read via ASIC registers.
Full VLAN dump & web support of new commands will be added later.