upgrade flutter environment version config

• upgrade config

environment:
  sdk: ">=2.12.0 <3.0.0"
  flutter: ">=1.20.0"

Migrate to null safety

• Migrate to null safety.

v0.3.5

fix something

v0.3.3

adapter: flutter version = 1.20.0

v0.3

Update "README"

v0.2

Support：argument

v0.1

Support：
1、route manager need't context object.
2、route manager with animation.