Add setLicenseKey method

[irgendeinich]

Resolves #268

Details

To match the iOS API we should also provide an option to manually set the license key.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, and samples/Catalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).