Use gateway specified on command line "docker network create"

[vladwing]

Currently dwgd doesn't set a default route and when it does set it using dwgd.route option, docker gets confused. Allow setting a default route using docker network create ... --gateway <IP>.

Copying from Issue #4, for context

The problem with the current implementation is that the routes are added as static routes, but docker doesn't understand it is 
dealing with a default route. Because of that, if you add another network to a container - for example a bridge, docker will see 
that the dwgd network doesn't provide a default route and will create a default route via the other network. You'll get an 
annoying message saying the container wasn't able to be created because 0.0.0.0/0 already exists. Currently dwgd doesn't use 
the gateway parameter, even though it is specified in the documentation as the way to create the network.

[vladwing]

Give me a bit of time to update the PR with the requested changes.