feat(hooks): Initial useOptimizelyFeature hook

[ajdaniel]

This is a React hook which implements the Optimizely Feature functionality.
It is very similar in functionality to the <OptimizelyFeature /> component.

usage:

import { useOptimizelyFeature } from '@optimizely/react-sdk'

function MyComponent() {
  const { isEnabled, loading } = useOptimizelyFeature('my_feature')
  if (loading) return null
  return isEnabled ? 'enabled!' : 'not-enabled'
}

Includes tests that were a copy of the Feature component tests, but with the hook instead.

[jamesopti]

@ajdaniel Thanks for opening this! I actually started working on this early last week and didn't notice your pull request until Friday. Sorry about that! The good news is we arrived at a very similar result.

Would love your feedback on #28 if you're interested in reviewing! I updated mine a little bit based yours.

In the future, if you can link a PR to an issue, it will help us make sure we associate the PR correctly with the issue (in this case, #6 )

Thanks again for contributing!

[ajdaniel]

Hey @jamesopti yeah I saw your PR open just after mine, I should have grabbed the issue 😞 .

I'll take a look at your PR and provide feedback. I'm happy to drop this PR in favour of yours.