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/plugins/builtin.py

@@ -36,6 +36,7 @@ class BasicAuthPlugin(BuiltinAuthPlugin):
     name = 'Basic HTTP auth'
     auth_type = 'basic'
 
+    # noinspection PyMethodOverriding
     def get_auth(self, username, password):
         return HTTPBasicAuth(username, password)
 
@@ -45,5 +46,6 @@ class DigestAuthPlugin(BuiltinAuthPlugin):
     name = 'Digest HTTP auth'
     auth_type = 'digest'
 
+    # noinspection PyMethodOverriding
     def get_auth(self, username, password):
         return requests.auth.HTTPDigestAuth(username, password)