Add ability to programmatically override environment defaults

[tbakerx]

Currently we can override environment defaults by setting the relevant environment variables, but it would be nice to programmatically set our own defaults

[jasonscharf]

Hi Tim,

These are the from the Next.js integration guide (with a modified Redis host) and work:

const redis = getRedisBackend({
    host: "foo",
});
const json = getJsonSiteBackend();

const defaultOptions: Partial<InstantBanditServerOptions> = {
    sessions: redis,
    metrics: redis,
    models: json,
};
export const server = getBanditServer(defaultOptions);

The order is code -> environment (if present) -> defaults from code (set in environment.ts as fallbacks).

Let me know if I'm missing something. I think moving the example and the precedence to the readme would clarify things, what do you think?