Loop configuration and exception prefix

[viniciusgerevini]

closes #33

• Added configuration to change default loop value.
• Added configuration to allow animations to be imported with opposite configuration.
• Prefix is removed from the animation name.

Here are some examples.

For configuration Loop animations = true and Loop exception prefix = "_":

Animations in Aseprite:  walk  run  _attack  _engage

Result in Godot:
- walk (loop = true)
- run (loop = true)
- attack (loop = false)
- engage (loop = false)

For configuration Loop animations = false and Loop exception prefix = "loop_":

Animations in Aseprite:  loop_walk  loop_run  attack engage

Result in Godot:
- walk (loop = true)
- run (loop = true)
- attack (loop = false)
- engage (loop = false)