Adds Stylus support

see #50

• Added Stylus ouput option to sprites task Thx to @alpadev
  • f25cb4e updates: README.md [@alpadev]
  • 894a702 adds: stylus generator [@alpadev]
  • c4554cb adds: stylus test case [@alpadev]
  • 84d2085 Don't output CSS unit for value 0 [@LaurentGoderre]