More langs, RTL lookup, and ePub constants

• Added file format constants for ePub
• Added RTL language lookup table in le_utils.constants.languages.RTL_LANG_CODES
• Added the following language codes:
  • be-tarask = Belarusian
  • hrx = Hunsrik
  • lua = Luba-Kasai
  • pms = Piedmontese
  • sco = Scots
  • rue = Rusyn