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

tests/test_auth_plugins.py

@@ -0,0 +1,89 @@
+from utils import http, HTTP_OK
+from httpie.plugins import AuthPlugin, plugin_manager
+
+# TODO: run all these tests in session mode as well
+
+# Basic auth encoded 'username' and 'password'
+BASIC_AUTH_HEADER_VALUE = 'Basic dXNlcm5hbWU6cGFzc3dvcmQ='
+BASIC_AUTH_URL = '/basic-auth/username/password'
+
+
+def dummy_auth(auth_header=BASIC_AUTH_HEADER_VALUE):
+
+    def inner(r):
+        r.headers['Authorization'] = auth_header
+        return r
+
+    return inner
+
+
+def test_auth_plugin_parse_false(httpbin):
+
+    class ParseFalseAuthPlugin(AuthPlugin):
+        auth_type = 'parse-false'
+        auth_parse = False
+
+        def get_auth(self, username=None, password=None):
+            assert username is None
+            assert password is None
+            assert self.raw_auth == BASIC_AUTH_HEADER_VALUE
+            return dummy_auth(self.raw_auth)
+
+    plugin_manager.register(ParseFalseAuthPlugin)
+    try:
+        r = http(
+            httpbin + BASIC_AUTH_URL,
+            '--auth-type', 'parse-false',
+            '--auth', BASIC_AUTH_HEADER_VALUE
+        )
+        assert HTTP_OK in r
+    finally:
+        plugin_manager.unregister(ParseFalseAuthPlugin)
+
+
+def test_auth_plugin_require_false(httpbin):
+
+    class RequireFalseAuthPlugin(AuthPlugin):
+        auth_type = 'require-false'
+        auth_require = False
+
+        def get_auth(self, username=None, password=None):
+            assert self.raw_auth is None
+            assert username is None
+            assert password is None
+            return dummy_auth()
+
+    plugin_manager.register(RequireFalseAuthPlugin)
+    try:
+        r = http(
+            httpbin + BASIC_AUTH_URL,
+            '--auth-type', 'require-false',
+        )
+        assert HTTP_OK in r
+    finally:
+        plugin_manager.unregister(RequireFalseAuthPlugin)
+
+
+def test_auth_plugin_prompt_false(httpbin):
+
+    class PromptFalseAuthPlugin(AuthPlugin):
+        auth_type = 'prompt-false'
+        prompt_password = False
+
+        def get_auth(self, username=None, password=None):
+            assert self.raw_auth == 'username:'
+            assert username == 'username'
+            assert password == ''
+            return dummy_auth()
+
+    plugin_manager.register(PromptFalseAuthPlugin)
+
+    try:
+        r = http(
+            httpbin + BASIC_AUTH_URL,
+            '--auth-type', 'prompt-false',
+            '--auth', 'username:'
+        )
+        assert HTTP_OK in r
+    finally:
+        plugin_manager.unregister(PromptFalseAuthPlugin)