v0.4: Independent fade_in/fade_out

Completed enhancements proposed in #12

New API methods:

• fade_out(options)
• fade_in(options)
  These new methods allow you to do half of a regular transition with the same options as change_scene()

New options:

• skip_fade_out : Bool
• skip_fade_in : Bool
  You can set these to true to skip parts of a transition

General internal rewrites, new deprecation warnings:

• you should replace the "no_scene_change" option for "skip_scene_change"
• you should remove the "type" option and FadeTypes enum entirely. For normal fade transitions, use "pattern": "fade" (this is the new default pattern)