v19 updates

[tjkessler]

Updates Plemmy to play nice with Lemmy 0.19.X instances.

• LemmyHttp object now optionally accepts a jwt token on __init__ if LemmyHttp.login method not called afterwards
• jwt token is no longer stored in the LemmyHttp object, it is passed directly to a requests.Session object
• plemmy.utils.create_session function accepts headers and jwt token, returns requests.Session object
  • used in LemmyHttp.__init__ and LemmyHttp.login
• plemmy.utils.get_handler, plemmy.utils.post_handler, and plemmy.utils.put_handler now accept a requests.Session object as their first arguments
• Updated all LemmyHttp methods to pass requests.Session object (LemmyHttp._session attribute) to updated utils functions

Version bump: 0.3.11 -> 0.4.0