Add optionaldependency on django-auth-ldap.

Advances #23.

pyproject.toml

@@ -35,6 +35,11 @@ django-sass-processor = "^0.7.3"
 libsass = "^0.19.2"
 colour = "^0.1.5"
 dynaconf = {version = "^2.0", extras = ["yaml", "toml", "ini"]}
+django-auth-ldap = { version = "^2.0", optional = true }
+
+[tool.poetry.extras]
+ldap = ["django-auth-ldap"]
+
 [tool.poetry.dev-dependencies]
 sphinx = "^2.1"
 sphinxcontrib-django = "^0.5.0"