lib: make the supported format names accessible

hledger-lib/Hledger/Read.hs

@@ -9,6 +9,7 @@ to import modules below this one.
 -}
 
 module Hledger.Read (
+       readFormatNames,
        -- * Journal reading API
        defaultJournalPath,
        defaultJournal,

hledger-lib/Hledger/Read/Util.hs

@@ -42,6 +42,9 @@ readers = [
  ,CsvReader.reader
  ]
 
+readFormatNames :: [StorageFormat]
+readFormatNames = map rFormat readers
+
 -- | Which readers are worth trying for this (possibly unspecified) format, filepath, and data ?
 readersFor :: (Maybe StorageFormat, Maybe FilePath, String) -> [Reader]
 readersFor (format,path,s) =