optparse compatibility - add float and complex

also documents the implementation quality
fixes #457

_pytest/config.py

@@ -552,11 +552,18 @@ class ArgumentError(Exception):
 
 
 class Argument:
-    """class that mimics the necessary behaviour of optparse.Option """
+    """class that mimics the necessary behaviour of optparse.Option
+
+    its currently a least effort implementation
+    and ignoring choices and integer prefixes
+    https://docs.python.org/3/library/optparse.html#optparse-standard-option-types
+    """
     _typ_map = {
         'int': int,
         'string': str,
-        }
+        'float': float,
+        'complex': complex,
+    }
     # enable after some grace period for plugin writers
     TYPE_WARN = False