Allow new beta version notifications

[waldekmastykarz]

Right now, Dev Proxy only notifies you on new minor/major versions. We should also allow folks to subscribe to latest beta versions and get a notification when we release one to try our latest features.

When implementing this, we need to check, if the current code can handle versions with labels (eg. -beta.1)

Tasks:

• Update new version check to support beta versions
• Add new config option (no arg) named newVersionNotification which can be set to latest (default) or beta
• Update docs