V0.2.0

Improvements:

• Added frame-level flag CGIF_FRAME_ATTR_HAS_ALPHA (for setting per-frame alpha channel #38)
• Added frame-level flag CGIF_FRAME_ATTR_HAS_SET_TRANS (allows the user to set which pixels are reused from the previous frame #38)

Project specific changes:

• Added checksum test for coverage test cases (45e6842)
• Refactored cgif (added internal and non-public raw API, #36)