Add property of progress indicator's background image

[YuyaAbo]

referred issue #14

add backgroundImage property at LiquidPullToRefresh.

Sample

# example/lib/main.dart
~~~
body: LiquidPullToRefresh(
        key: _refreshIndicatorKey,
        onRefresh: _handleRefresh,
+       backgroundImage: Image.network("http://placehold.jp/150x150.png").image,
~~~

I would be happy if you could check it. 🙏

[codecov]

Codecov Report

Merging #34 into master will decrease coverage by 0.32%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   86.06%   85.74%   -0.33%     
==========================================
  Files           3        3              
  Lines         445      449       +4     
==========================================
+ Hits          383      385       +2     
- Misses         62       64       +2     

Impacted Files	Coverage Δ
lib/src/circular_progress.dart	87.50% <33.33%> (-3.62%)	⬇️
lib/liquid_pull_to_refresh.dart	87.98% <100.00%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63ed32a...f5f4b5b. Read the comment docs.