Add support for animated transitions between the various loading states

[SamCosta1]

This attempts to resolve #193 .

Adds a transition configuration object that when provided will be used to animate between states (e.g placeholder to loaded/error state). If omitted there's no animation. Values in the configuration are just forwarded to the AnimatedSwitcher widget which does the animation.

[SamCosta1]

CC @dnfield

[zanderso]

Hi there, The Flutter team is now maintaining these packages in the flutter/packages repository at packages/vector_graphics and packages/vector_graphics_compiler. We'd appreciate it if you could direct this PR to those locations instead. Thanks!