Add unique_track method

[marckohlbrugge]

Proposal for #107

Adds an unique_track method to ensure uniqueness when tracking the same event multiple times, by adding a ULID.

Ideally, we would add this to the existing track method, but doing so in a backwards-compatible way requires jumping through various hoops. I figured a separate method might be simpler.

[marckohlbrugge]

Bump!

[lukeasrodgers]

I also need this functionality, and this implementation LGTM.