Added netrc support for auth plugins.

Enabled for --auth-type=basic and digest, 3rd parties may opt in.

This closes #718, closes #719, closes #852, and also closes #934

httpie/plugins/base.py

@@ -35,13 +35,22 @@ class AuthPlugin(BasePlugin):
     # Set this to `False` to disable the parsing and error handling.
     auth_parse = True
 
+    # Set to `True` to make it possible for this auth
+    # plugin to acquire credentials from the user’s netrc file(s).
+    # It is used as a fallback when the credentials are not provided explicitly
+    # through `--auth, -a`. Enabling this will allow skipping `--auth, -a`
+    # even when `auth_require` is set `True` (provided that netrc provides
+    # credential for a given host).
+    netrc_parse = False
+
     # 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.
+    # `get_auth()` gets called. If the credentials came from a netrc file,
+    # then this is `None`.
     raw_auth = None
 
     def get_auth(self, username=None, password=None):

httpie/plugins/builtin.py

@@ -22,6 +22,7 @@ class HTTPBasicAuth(requests.auth.HTTPBasicAuth):
         See https://github.com/jakubroztocil/httpie/issues/212
 
         """
+        # noinspection PyTypeChecker
         request.headers['Authorization'] = type(self).make_header(
             self.username, self.password).encode('latin1')
         return request
@@ -36,6 +37,7 @@ class HTTPBasicAuth(requests.auth.HTTPBasicAuth):
 class BasicAuthPlugin(BuiltinAuthPlugin):
     name = 'Basic HTTP auth'
     auth_type = 'basic'
+    netrc_parse = True
 
     # noinspection PyMethodOverriding
     def get_auth(self, username: str, password: str) -> HTTPBasicAuth:
@@ -43,9 +45,9 @@ class BasicAuthPlugin(BuiltinAuthPlugin):
 
 
 class DigestAuthPlugin(BuiltinAuthPlugin):
-
     name = 'Digest HTTP auth'
     auth_type = 'digest'
+    netrc_parse = True
 
     # noinspection PyMethodOverriding
     def get_auth(