[mangadex] implement login with client credentials

unless the legacy method still works for your account,
login now requires 4 values:

- client-id
- client-secret
- username
- password

where 'client-id' and 'client-secret' are the credentials
of a personal API client.

> Public clients are not yet available.

docs/gallery-dl.conf

@@ -409,6 +409,8 @@
         },
         "mangadex":
         {
+            "client-id"    : "",
+            "client-secret": "",
             "username": "",
             "password": "",