Add Feature Flags support

[yakkomajuri]

Dear Reviewer

Given prettier was run here, feature-flags.js and test.js are the places to focus most of the energy on.

What this does

1. Adds support for feature flags
2. Make everything prettier

What this does not do

1. Refactor the existing codebase - I was originally doing this but decided to leave this up to a separate PR. Things like consolidating all the requests, etc.

How the feature flags work

This follows the posthog-python implementation almost exactly.

Follow-up work

1. Documentation
2. Writing up a feature flags spec - indeed the posthog-python code somewhat serves as this, but we need a place that at least tells us how to correctly calculate simple flags so we don't end up with diverging implementations. I spent some time playing around with crypto to get this right.
3. Refactor

[Twixes]

Please please never mix code formatting changes with behavior changes. I made #30 to make this reviewable. Now this PR only includes material updates to the code.
A couple of comments besides that but overall this looks reasonable.

[yakkomajuri]

@Twixes yeah I do apologize for the prettier changes coming in this PR

[yakkomajuri]

Ready to go

[yakkomajuri]

Also writing docs now

[Twixes]

Letsgooo

[weyert]

Awesome 🥇