servers - variable substitutions not supported

[fogine]

OAS: v3
the following configuration should be supported:

{
    servers: [
        {
             url: '{protocol}{host}{basePath}',
             variables: {
                 protocol: {default: 'http://'},
                 host: {default: '127.0.0.1'},
                 basePath: {default: ''}
        }
    ]
}

currently it renders endpoints as {protocol}{host}{basePath} instead of http://127.0.0.1

I might make a PR.

[RomanHotsiy]

@fogine go for it 🏃!
Thanks!