add flex center

[GeekTree0101]

FlexCenter(overlay, option: .XY) { center in
  FlexItem(center, view: self.indexLabel)
}

or

FlexCenter(
  overlay,
  option: .XY,
  view: self.indexLabel
)

option Y

option X

option XY

[GeekTree0101]

KarrotFlex should consider using a snapshot test. :(..

[GeekTree0101]

i just add snapshot testing env #5

[GeekTree0101]

After merging it first, I will take it in the direction of supplementing the snapshot test.