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/base.py

@@ -17,13 +17,36 @@ class AuthPlugin(BasePlugin):
 
     See <https://github.com/jkbrzt/httpie-ntlm> for an example auth plugin.
 
+    See also `test_auth_plugins.py`
+
     """
     # The value that should be passed to --auth-type
     # to use this auth plugin. Eg. "my-auth"
     auth_type = None
 
-    def get_auth(self, username, password):
+    # Set to `False` to make it possible to invoke this auth
+    # plugin without requiring the user to specify credentials
+    # through `--auth, -a`.
+    auth_require = True
+
+    # By default the `-a` argument is parsed for `username:password`.
+    # Set this to `False` to disable the parsing and error handling.
+    auth_parse = True
+
+    # If both `auth_parse` and `prompt_password` are set to `True`,
+    # and the value of `-a` lacks the password part,
+    # then the user will be prompted to type the password in.
+    prompt_password = True
+
+    # Will be set to the raw value of `-a` (if provided) before
+    # `get_auth()` gets called.
+    raw_auth = None
+
+    def get_auth(self, username=None, password=None):
         """
+        If `auth_parse` is set to `True`, then `username`
+        and `password` contain the parsed credentials.
+
         Return a ``requests.auth.AuthBase`` subclass instance.
 
         """

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)

httpie/plugins/manager.py

@@ -24,6 +24,9 @@ class PluginManager(object):
         for plugin in plugins:
             self._plugins.append(plugin)
 
+    def unregister(self, plugin):
+        self._plugins.remove(plugin)
+
     def load_installed_plugins(self):
         for entry_point_name in ENTRY_POINT_NAMES:
             for entry_point in iter_entry_points(entry_point_name):