This extends the `AuthPlugin` API by the following attributes:
* `auth_require`: set to `False` to make `--auth, -a` optional
* `auth_parse`: set to `False` to disable `username:password` parsing
(access the raw value passed to `-a` via `self.raw_auth`).
* `prompt_password`: set to`False` to disable password prompt when
no password provided (only relevant when `auth_parse == True`)
These changes should be 100% backwards-compatible.
What needs more testing is auth support in sessions.
Close#433Close#431Close#378
Ping teracyhq/httpie-jwt-auth#3
