Add -bip flag: allow specification of dynamic bridge IP via CIDR

e.g.:

```
docker -d -bip "10.10.0.1/16"
```

If set and valid, use provided in place of trial and error from pre-defined array in network.go.
Mutually exclusive of -b option.
Merge pull request #3208 from WarheadsSE/bridgeip

Add -bip flag: allow specification of dynamic bridge IP via CIDR