fix tags with spaces

add a filter that replaces tags e.g.
"tag name" → {name: "tag name", slug: "tag_name"}

blog-meta/lib/Misc.hs

@@ -3,6 +3,10 @@ module Misc where
 import qualified System.Console.GetOpt as GetOpt
 import System.Environment
 import System.Exit
+import Data.Text (Text)
+import qualified Data.Text as Text
+import Data.Char (isAlphaNum, isAscii, toLower)
+import Text.Pandoc.Definition
 
 -- | exception on 'Left'
 unwrap :: Show a => FilePath -> Either a b -> IO b
@@ -20,3 +24,34 @@ getOptionsWith hdr mkDef descrs = do
       putStrLn $ GetOpt.usageInfo (hdr prog) descrs
       exitFailure
 
+makeSlug :: Text -> Text
+makeSlug name = Text.map toSlugChar name where
+  toSlugChar c
+    | isAlphaNum c && isAscii c || c == '-' = toLower c
+    | otherwise                             = '_'
+
+
+toTextList :: MetaValue -> Maybe [Text]
+toTextList (MetaList vs) =  traverse toText vs
+toTextList _ = Nothing
+
+toText :: MetaValue -> Maybe Text
+toText (MetaString str) = Just str
+toText (MetaInlines is) = foldMap inlineText is
+toText (MetaBlocks bs) = foldMap blockText bs
+toText _ = Nothing
+
+inlineText :: Inline -> Maybe Text
+inlineText (Str txt)         = Just txt
+inlineText Space             = Just " "
+inlineText SoftBreak         = Just " "
+inlineText LineBreak         = Just " "
+inlineText (RawInline _ txt) = Just txt
+inlineText _                 = Nothing
+
+blockText :: Block -> Maybe Text
+blockText (Plain is)       = foldMap inlineText is
+blockText (Para  is)       = foldMap inlineText is
+blockText Null             = Just ""
+blockText (RawBlock _ txt) = Just txt
+blockText _                = Nothing