Add center align

Author: nobu

lib/rdoc/markdown.kpeg

@@ -1214,8 +1214,10 @@ TableLine = ( ( TableAlign:align1 TableAlign2*:aligns {[align1, *aligns] } ):lin
 	    { line }
 TableAlign2 = "|" @Sp TableAlign
 TableAlign = < /:?-+:?/ > @Sp
-              { text.start_with?(":") ? :left :
-                text.end_with?(":") ? :right : nil
+              {
+                text.start_with?(":") ?
+                (text.end_with?(":") ? :center : :left) :
+                (text.end_with?(":") ? :right : nil)
               }
 
 DefinitionList = &{ definition_lists? }

test/rdoc/test_rdoc_markdown.rb

@@ -1065,14 +1065,14 @@ def test_gfm_table
   def test_gfm_table_2
     doc = parse <<~'MD'
     | Cmd | Returns | Meaning
-    ----- | ------- | -------
+    ----- | :-----: | -------
     |"b"  | boolean | True if file1 is a block device
     "c"   | boolean | True if file1 is a character device
     |"\|" | boolean | escaped bar \| test
     MD
 
     head = %w[Cmd Returns Meaning]
-    align = [nil, nil, nil]
+    align = [nil, :center, nil]
     body = [
       ['"b"', 'boolean', 'True if file1 is a block device'],
       ['"c"', 'boolean', 'True if file1 is a character device'],