And env based endpoint/user/etc.

[jreadey]

Pull endpoint, user, password, etc. from env variable or config file if not set explicitly.

[jreadey]

This is implemented using the following environment variables:

• H5SERV_ENDPOINT -- endpoint for the server, e.g. "http://127.0.0.1:5000"
• H5SERV_USERNAME -- username that will be sent with the Authorization header
• H5SERV_PASSWORD -- password that will be sent with the Authorization header

These can be overridden with named options in the File constructor.