Option to not run event loop on import

[phil65]

Currently tokencost can not get imported lazily inside an async application properly because it tries to run its own eventloop for updating the cost data.
I think that it should either:

1. check for an existing eventloop and use that one if existing
   or, better
2. allow an option to not update at all on import (perhaps via environment variable? That would allow to keep bw compat)

I think in general doing IO work on an import is not really ideal, would love if that could be reconsidered.

Thank you!