v0.2.1

• Replaced enums::Tone::Xyz with enums::Tone::Lab, which seems to work better for brightness and contrast
  adjustments

• Added parallel feature for enabling multithreading

• Added PixelIter struct for iterating over an image by pixels

• Added morphology module functions:

  • erode()
  • dilate()
  • majority()
  • open()
  • close()
  • gradient()

• Fixed bug in tone::brightness() and tone::contrast()