add setOAuthCredentials function to supply the client with existing oAuth token and secret

[morriswchris]

Rather than needing to supply the client a username/password each time, allow the client to be build with an existing oAuth token and secret. This allows for token caching vs needing the user to supply a username and password to the application each time.

Sample:

const client = new Instapaper(CONSUMER_KEY, CONSUMER_SECRET);
client.setOAuthCredentials(TOKEN, SECRET);