v0.5.0: Release 0.5.0

• Changing output asserts
  • .prints -> .stdout().contains
  • .prints_exactly -> .stdout().is
  • .prints_error -> .stderr().contains
  • .prints_error_exactly -> .stderr().is
• Add negative output assertions
  • .stdout().isnt
  • .stdout().doesnt_contain
• Output assertions are now anded together rather than replacing each
  other.