Allow specification of port with environment variable.

This lets me listen on all adapters, and specify the port, which makes working in devstack easier.
Thanks for the PR.  Would you mind changing this to `FLASK_LTI_PORT` to avoid possible collisions, and also make the `host` settable by environment with something like `FLASK_LTI_HOST` with a default of `localhost`?  I definitely want it configurable, but would prefer it to not listen to the world by default.