Implement "dialog" form method value (#34)

See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form#method

Author: Hi-Angel

src/DOM/HTML/Indexed/FormMethod.purs

@@ -5,6 +5,7 @@ import Prelude
 data FormMethod
   = POST
   | GET
+  | Dialog
 
 derive instance eqFormMethod :: Eq FormMethod
 derive instance ordFormMethod :: Ord FormMethod
@@ -13,3 +14,4 @@ renderFormMethod :: FormMethod -> String
 renderFormMethod = case _ of
   POST -> "post"
   GET -> "get"
+  Dialog -> "dialog"