transparent firewall, 2nd attempt

(The first attempt was #337.) The key difference is a custom Rubocop configuration file that permits up to 200 lines of code per class definition (default is 100). This addresses the only Rubocop warning that this code had and it seems to be the only way to address it.

I propose considering a margin higher than 100 lines of code per class for all Trema examples, as typical Trema application code just doesn't fit 100 lines of code combined with 80-columns max line length. For instance, a `send_flow_mod_add()` call takes 4 and sometimes more lines of code:
```
    send_flow_mod_add(
      @dpid, priority: prio,
             match: Match.new(in_port: port_no, dl_type: 0x0800, nw_dst: pfx),
             actions: [SendOutPort.new(@inspect_port_no)])
```
Please review.