Change Package name for the NewCfClient function.

[davejohnston]

Currently to create a new CfClient we have to import the package 'client'. And call NewCfClient. This isn't very go idiomatic.
We probably need to support the API at this stage, but should create a package called harness or flags and define the NewCfClient in it. That way when instantiating the client the user would call

client := flags.NewCfClient(sdkKey string, options ...ConfigOption)

We can have the current NewCfClient call the new one, to maintain backwards compatability i.e.
package client

func NewCfClient(sdkKey string, options ...ConfigOption) (*CfClient, error) {
    return flags.NewCfClient(sdkKey, options)
}

[enver-bisevac]

@davejohnston What do you think about this ff := client.New(sdkKey string, options ...ConfigOption) it doesn require any breaking change just add New function. Later we can introduce new package ffclient in root of the project
`client := ffclient.New(sdkKey, options)