v0.3.1

• Updates for CRAN resubmission:
  • The list representing game state now has an S3 class - "wordler".
  • All exported functions now have a documented return value.
  • All functions which accept and return a "wordler" object are documented
    regarding their effects on the "wordler" object.
  • Documented the wordler class (in the constructor new_wordler()).
• Fixed issue with detection of letters known to be in, or known not to be in,
  the target word.
• make_new_game() superseded by new_wordler().