adding endpoint option to init function

I have a use case where all requests must flow back through the original domain (no XSS allowed)

to support using logentries from the client side i have added a proxy route on the original domain and need to be able to set a custom endpoint for le so that it can hit this endpoint instead of directly connecting to `js.logentires.com`

happy to take suggestions on ways to improve this, but its definitely a feature i would appreciate seeing in the library if others agree it would be useful.