Add support for Subscriber Tagging

[dwanjuki]

Is your feature request related to a problem? Please describe.
There is currently no documented way to add Tags to subscribers in MailPoet using our integration.
More on Subscriber Tags

Describe the solution you'd like
A code snippet to add tags to subscribers in MailPoet on membership level changes.

[andrewlimaza]

@dwanjuki Thanks for this, can you submit a doc update/snippet request for P2. Code snippets should be tracked there.