Add methods to help with user self-service opt in (labs)

[slumos]

Two methods added:

• get_user_toggles retrieves all toggles evaluated for the given user,
• add_user_override does as the name implies.

We're using this in production for our Labs feature.

Thanks LaunchDarkly!

[jkodumal]

@slumos I've put up a more efficient version of this in #51

The method names have changed a bit to be more consistent with other SDKs, but these new methods should return the same types.

[slumos]

Closing since LaunchDarkly provided much better methods.