FormatOptions: added lineIdPrefix. API change.

This is needed because a web page may contain several
code samples, and we need to make sure that the ids on
lines are unique.

See jgm/pandoc#4031.

src/Skylighting/Types.hs

@@ -370,6 +370,7 @@ data FormatOptions = FormatOptions{
        , titleAttributes  :: Bool     -- ^ Html titles with token types
        , codeClasses      :: [Text]   -- ^ Additional classes for Html code tag
        , containerClasses :: [Text]   -- ^ Additional classes for Html container tag
+       , lineIdPrefix     :: Text     -- ^ Prefix for id attributes on lines
        } deriving (Show, Read, Eq, Ord, Data, Typeable, Generic)
 
 instance Binary FormatOptions
@@ -383,4 +384,5 @@ defaultFormatOpts = FormatOptions{
                     , titleAttributes = False
                     , codeClasses = []
                     , containerClasses = []
+                    , lineIdPrefix = ""
                     }