Provide control over setting the client_id in common event

[adatzer]

Currently the client_id is populated by domain_userid or by mobile_context's user id, if exists. There are may be events where none of them exists (e.g. a server-side tracked event), so this results in client_id being undefined. Since client_id is a property that downstream tags could count upon, we could provide an option to override the default client_id setting logic to better support those use cases.

[adatzer]

This has been included in #22 and resolved in latest v0.5.0. Closing.