Added onFinished callback to TextLiquidFill

[ertgrulll]

Added onFinished callback to TextLiquidFill, readme and example updated according to change.

[SirusCodes]

@aagarwal1012 I guess it will be a great addition to all the animation not only TextLiquidFill, so it would be great if it would be included in AnimatedTextKit class so every animation will get this.
I just wanted to know that if there is any way we already support this?

[ertgrulll]

AnimatedTextKit already has onFinished that triggering on children animations end. But TextLiquidFill is not using as AnimatedTextKit child. I am routing user to another screen on this animation end, so I was need to listen that. I think just TextLiquidFill need this property 🙄

[codecov]

Codecov Report

Merging #262 (f64bfb8) into master (24c2592) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   95.38%   95.44%   +0.06%     
==========================================
  Files          10       10              
  Lines         455      461       +6     
==========================================
+ Hits          434      440       +6     
  Misses         21       21              

Impacted Files	Coverage Δ
lib/src/text_liquid_fill.dart	100.00% <100.00%> (ø)

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