Add generic attributes extension.

This allows attributes to be added to any block or inline
element, in principle.  (Though in many cases this will be
done by adding a Div or Span container, since pandoc's
AST doesn't have a slot for attributes for most elements.)

Currently this is only possible with the commonmark and gfm
readers.

Add `Ext_attributes` constructor for `Extension` [API change].

MANUAL.txt

@@ -4933,6 +4933,37 @@ in pandoc, but may be enabled by adding `+EXTENSION` to the format
 name, where `EXTENSION` is the name of the extension.  Thus, for
 example, `markdown+hard_line_breaks` is Markdown with hard line breaks.
 
+#### Extension: `attributes` ####
+
+Allows attributes to be attached to any inline or block-level
+element.  The syntax for the attributes is the same as that
+used in [`header_attributes`][Extension: `header_attributes`].
+
+- Attributes that occur immediately after an inline
+  element affect that element.  If they follow a space, then they
+  belong to the space.  (Hence, this option subsumes
+  `inline_code_attributes` and `link_attributes`.)
+- Attributes that occur immediately before a block
+  element, on a line by themselves, affect that
+  element.
+- Consecutive attribute specifiers may be used,
+  either for blocks or for inlines.  Their attributes
+  will be combined.
+- Attributes that occur at the end of the text of
+  a Setext or ATX heading (separated by whitespace
+  from the text) affect the heading element. (Hence, this
+  option subsumes `header_attributes`.)
+- Attributes that occur after the opening fence
+  in a fenced code block affect the code block element. (Hence,
+  this option subsumes `fenced_code_attributes`.)
+- Attributes that occur at the end of a reference
+  link definition affect links that refer to that
+  definition.
+
+Note that pandoc's AST does not currently allow attributes
+to be attached to arbitrary elements.  Hence a Span or Div
+container will be added if needed.
+
 #### Extension: `old_dashes` ####
 
 Selects the pandoc <= 1.8.2.1 behavior for parsing smart dashes:

src/Text/Pandoc/Extensions.hs

@@ -150,6 +150,7 @@ data Extension =
     | Ext_tex_math_single_backslash  -- ^ TeX math btw \(..\) \[..\]
     | Ext_yaml_metadata_block -- ^ YAML metadata block
     | Ext_gutenberg           -- ^ Use Project Gutenberg conventions for plain
+    | Ext_attributes          -- ^ Generic attribute syntax
     deriving (Show, Read, Enum, Eq, Ord, Bounded, Data, Typeable, Generic)
 
 -- | Extensions to be used with pandoc-flavored markdown.
@@ -447,11 +448,8 @@ getAllExtensions f = universalExtensions <> getAll f
     , Ext_fenced_divs
     , Ext_bracketed_spans
     , Ext_raw_attribute
-    , Ext_inline_code_attributes
-    , Ext_fenced_code_attributes
-    , Ext_link_attributes
-    , Ext_header_attributes
     , Ext_implicit_header_references
+    , Ext_attributes
     ]
   getAll "commonmark"      = getAll "gfm"
   getAll "org"             = autoIdExtensions <>

src/Text/Pandoc/Readers/CommonMark.hs

@@ -48,10 +48,7 @@ readCommonMark opts s = do
          [ fencedDivSpec | isEnabled Ext_fenced_divs opts ] ++
          [ bracketedSpanSpec | isEnabled Ext_bracketed_spans opts ] ++
          [ rawAttributeSpec | isEnabled Ext_raw_attribute opts ] ++
-         [ attributesSpec | isEnabled Ext_link_attributes opts ||
-                            isEnabled Ext_inline_code_attributes opts ||
-                            isEnabled Ext_fenced_code_attributes opts ||
-                            isEnabled Ext_header_attributes opts ] ++
+         [ attributesSpec | isEnabled Ext_attributes opts ] ++
          [ pipeTableSpec | isEnabled Ext_pipe_tables opts ] ++
          [ autolinkSpec | isEnabled Ext_autolink_bare_uris opts ] ++
          [ emojiSpec | isEnabled Ext_emoji opts ] ++