Add an option to conceal preformatted text blocks

This merges PR #641.

doc/vimwiki.txt

@@ -2887,6 +2887,26 @@ URLs and hides markers and URL for links that have a description.
 Default: 2
 
 
+------------------------------------------------------------------------------
+*g:vimwiki_conceal_pre*
+
+Conceal preformatted text markers. For example,
+>
+    {{{python
+    def say_hello():
+        print("Hello, world!")
+    }}}
+>
+would appear as simply
+>
+    def say_hello():
+        print("Hello, world!")
+>
+in your wiki file.
+
+Default: 0
+
+
 ------------------------------------------------------------------------------
 *g:vimwiki_autowriteall*