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.

docs/sources/commandline/cli.rst

@@ -30,6 +30,7 @@ To list available commands, either run ``docker`` with no parameters or execute
       -H=[unix:///var/run/docker.sock]: Multiple tcp://host:port or unix://path/to/socket to bind in daemon mode, single connection otherwise
       -api-enable-cors=false: Enable CORS headers in the remote API
       -b="": Attach containers to a pre-existing network bridge; use 'none' to disable container networking
+      -bip="": Use the provided CIDR notation address for the dynamically created bridge (docker0); Mutually exclusive of -b
       -d=false: Enable daemon mode
       -dns="": Force docker to use specific DNS servers
       -g="/var/lib/docker": Path to use as the root of the docker runtime