Create demo app, add new initializer

[bdbergeron]

Add a new demo app to the Xcode project, showcasing various RemoteImage usages.

Also adds a new RemoteImage initializer that allows for simple usage with just a URL but with a closure to modify the loaded image:

RemoteImage(url: imageURL) {
  $0.resizable().scaledToFit()
}

[codecov]

Codecov Report

Merging #7 (2e5495e) into main (0d68275) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #7   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         7    +3     
  Lines          213       239   +26     
=========================================
+ Hits           213       239   +26     

Flag	Coverage Δ
iOS	100.00% <100.00%> (ø)
macOS	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
Sources/RemoteImage/RemoteImageViewModel.swift	100.00% <ø> (ø)
Sources/RemoteImage/RemoteImage+Helpers.swift	100.00% <100.00%> (ø)
...ources/RemoteImage/RemoteImage+InitWithCache.swift	100.00% <100.00%> (ø)
Sources/RemoteImage/RemoteImage.swift	100.00% <100.00%> (ø)
...moteImage/RemoteImageViewModel+InitWithCache.swift	100.00% <100.00%> (ø)

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