pandoc.mediabag module: add function empty

Function `pandoc.mediabag.empty` was added. It allows to clean-out the media
bag, removing all entries.

doc/lua-filters.md

@@ -2448,9 +2448,14 @@ The module is loaded as part of module `pandoc` and can either be
 accessed via the `pandoc.mediabag` field, or explicitly required,
 e.g.:
 
-
     local mb = require 'pandoc.mediabag'
 
+### empty {#mediabag-empty}
+
+`empty ()`
+
+Clear-out the media bag, deleting all items.
+
 ### insert {#mediabag-insert}
 
 `insert (filepath, mime_type, contents)`

src/Text/Pandoc/Lua/Module/MediaBag.hs

@@ -34,6 +34,7 @@ import qualified Text.Pandoc.MediaBag as MB
 pushModule :: Lua NumResults
 pushModule = do
   Lua.newtable
+  addFunction "empty" empty
   addFunction "insert" insertMediaFn
   addFunction "items" items
   addFunction "lookup" lookupMediaFn
@@ -60,6 +61,11 @@ setCommonState st = do
 modifyCommonState :: (CommonState -> CommonState) -> Lua ()
 modifyCommonState f = getCommonState >>= setCommonState . f
 
+-- | Delete all items from the media bag.
+empty :: Lua NumResults
+empty = 0 <$ modifyCommonState (\st -> st { stMediaBag = mempty })
+
+-- | Insert a new item into the media bag.
 insertMediaFn :: FilePath
               -> Optional MimeType
               -> BL.ByteString