[FEATURE] Tracking property Pattern validation

[hitherejoe]

We'e recently started integrating Typewriter @ Buffer - it's been frictionless to setup, thanks for the great work!

I'm currently working on the Android side of things, one thing that I think would be great to have in place is pattern validation. For example, in some of our tracking events we have the following declared:

- name: product
  ...
  type: string
  pattern: publish|analyze|remix

it would be great for Typewriter to throw an error if the value being sent does not match the required value. Are there any plans to add this? I was thinking of taking a look at contributing this, but wasn't sure if it was in the pipeline already 🙂

Thanks!