Extend auth plugin API

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 #433
Close #431
Close #378
Ping teracyhq/httpie-jwt-auth#3

httpie/client.py

@@ -145,11 +145,6 @@ def get_requests_kwargs(args, base_headers=None):
     headers.update(args.headers)
     headers = finalize_headers(headers)
 
-    credentials = None
-    if args.auth:
-        auth_plugin = plugin_manager.get_auth_plugin(args.auth_type)()
-        credentials = auth_plugin.get_auth(args.auth.key, args.auth.value)
-
     cert = None
     if args.cert:
         cert = args.cert
@@ -168,7 +163,7 @@ def get_requests_kwargs(args, base_headers=None):
         }.get(args.verify, args.verify),
         'cert': cert,
         'timeout': args.timeout,
-        'auth': credentials,
+        'auth': args.auth,
         'proxies': dict((p.key, p.value) for p in args.proxy),
         'files': args.files,
         'allow_redirects': args.follow,