Fix --optional option in add command

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 - Fixed Python version retrieval inside virtualenvs.
 - Fixed optional dependencies being set as required in sdist.
+- Fixed `--optional` option in the `add` command not being used.
 
 
 ## [0.6.3] - 2018-03-20

poetry/console/commands/add.py

@@ -53,6 +53,12 @@ If you do not specify a version constraint, poetry will choose a suitable one ba
             parser.parse_constraints(constraint)
 
         for name, constraint in requirements.items():
+            if self.option('optional'):
+                constraint = {
+                    'version': constraint,
+                    'optional': True
+                }
+
             poetry_content[section][name] = constraint
 
         # Write new content