[WIP] add useTreatment hook

[besh]

React SDK

What did you accomplish?

Added a useTreatment hook that takes in a single split. Addresses https://github.com/splitio/react-client/issues/69

How do we test the changes introduced in this PR?

Added tests to verify the new hook. You can test locally via npm-link or yalc in a project repo. One test will fail until I modify further (hence WIP).

Extra Notes

Initially, I wanted to create a useTreatment hook that used client.getTreatment. However, the existing useTreatments hook is using getTreatmentsWithConfig under the hood, so I chose to follow the existing convention. Shouldn't it instead be using getTreatements and then have a useTreatementsWithConfig hook that uses getTreatmentsWithConfig? I understand that at this point, it would be a version breaking change.

The end API that I'd like is

const treatment = useTreatment('my_split');

if (treatment === 'on') {
  // do something
}

However, because I'm following the existing convention of the useTreatments hook, the resulting usage is instead

const treatmentResult = useTreatment('my_split');

if (treatmentResult.treatment === 'on') {
  // do something
}

Would you be okay if I added two hooks?

• useTreatment -> uses client.getTreatment
• useTreatmentWithConfig -> uses client.getTreatmentWithConfig

These additions wouldn't be version breaking as they are additive, but it wouldn't follow the existing convention set in place. Looking for feedback and thoughts here!