Allow specifying string value in metadata using !!literal tag.

This is experimental.  Normally metadata values are interpreted
as markdown, but if the !!literal tag is used they will be interpreted
as plain strings.

We need to consider whether this can still be implemented if
we switch back from HsYAML to yaml for performance reasons.

MANUAL.txt

@@ -3914,7 +3914,8 @@ from the main markdown input document.
 
 Metadata will be taken from the fields of the YAML object and added to any
 existing document metadata.  Metadata can contain lists and objects (nested
-arbitrarily), but all string scalars will be interpreted as Markdown.  Fields
+arbitrarily), but all string scalars will be interpreted as
+Markdown, unless marked with the YAML tag `!!literal`.  Fields
 with names ending in an underscore will be ignored by pandoc.  (They may be
 given a role by external processors.)  Field names must not be
 interpretable as YAML numbers or boolean values (so, for
@@ -3945,6 +3946,11 @@ when the field contains blank lines or block-level formatting:
       This is the abstract.
 
       It consists of two paragraphs.
+
+    # This is a YAML comment; it will be ignored.
+    # The following field is marked as literal and will be
+    # interpreted as a literal string rather than Markdown:
+    catalog_number: !!literal '*abc123<f>*'
     ...
 
 Template variables will be set automatically from the metadata.  Thus, for

src/Text/Pandoc/Readers/Metadata.hs

@@ -99,10 +99,12 @@ yamlToMetaValue pBlocks (YAML.Scalar _ x) =
        YAML.SBool b      -> return $ return $ MetaBool b
        YAML.SFloat d     -> return $ return $ MetaString $ tshow d
        YAML.SInt i       -> return $ return $ MetaString $ tshow i
-       YAML.SUnknown _ t ->
-         case checkBoolean t of
-           Just b        -> return $ return $ MetaBool b
-           Nothing       -> toMetaValue pBlocks t
+       YAML.SUnknown tag t
+         | (T.takeWhileEnd (/= ':') <$> YE.tagToText tag) ==
+           Just "literal" -> return $ return $ MetaString t
+         | otherwise -> case checkBoolean t of
+                           Just b    -> return $ return $ MetaBool b
+                           Nothing   -> toMetaValue pBlocks t
        YAML.SNull        -> return $ return $ MetaString ""
 
 yamlToMetaValue pBlocks (YAML.Sequence _ _ xs) = do

test/command/literal-tag.md

@@ -0,0 +1,10 @@
+```
+% pandoc -t native -s
+---
+hi: !!literal '*ook*'
+there: '*ook*'
+...
+^D
+Pandoc (Meta {unMeta = fromList [("hi",MetaString "*ook*"),("there",MetaInlines [Emph [Str "ook"]])]})
+[]
+```