Provide custom way of rendering progress bar + typing fixes

[ffMathy]

This will make it possible to use my own progress bar if needed.

Also fixed typings of style prop (which in turn removed linter warning), and also improved the type of fetch to be more precise.

[codecov]

Codecov Report

Merging #6 into master will decrease coverage by 4.83%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #6 +/- ## =========================================== - Coverage 100.00% 95.16% -4.84%  =========================================== Files 1 1 Lines 48 62 +14 Branches 3 14 +11 =========================================== + Hits 48 59 +11  - Misses 0 3 +3 

Impacted Files	Coverage Δ
src/index.tsx	95.16% <0.00%> (-4.84%)	⬇️

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[ffMathy]

Codecov fails, but I think it is still reasonable. What do you think?

[MrHus]

Codecov fails, but I think it is still reasonable. What do you think?

Good idea to support a render function.

Two things:

1. The test coverage should be 100% and test the render function.

2. We should add a children render prop as well

[MrHus]

React.ReactNode should be the return type of the render.

[ffMathy]

Can you elaborate what you mean by the children prop?

[MrHus]

Can you elaborate what you mean by the children prop?

@ffMathy

That we support both: render and children render function props:

<ProgressBar render={mode => <span>{mode}</span}/>

<ProgressBar>{ mode => <span>{mode}</span}</ProgressBar>