Expose a way to await the initial datafile download in initialize

[jrbarnard]

Currently if you run initialise then immediately run a route using the middleware you get errors due to the datafile not yet being downloaded.

It would be good to be able to await the initialiser otherwise you can't confirm that the middleware is ready
e.g:

import optimizelyExpress from '@optimizely/express';
const optimizely = optimizelyExpress(config);

await optimizely.onReady();

// Use the middleware

Errors:

[OPTIMIZELY] - ERROR 2020-11-04T14:25:26.653Z PROJECT_CONFIG_MANAGER: You must provide at least one of sdkKey or datafile. Cannot start Optimizely
Run middleware
[OPTIMIZELY] - ERROR 2020-11-04T14:25:26.662Z OPTIMIZELY: Optimizely object is not valid. Failing isFeatureEnabled.