Add config to plugin callbacks

[Blackbaud-SteveBrush]

This feature provides the skyPagesConfig object to plugin preload and postload hooks.

module.exports = {
  preload: (content, resourcePath, skyAppConfig) => {
    return content;
  }
};

[codecov-io]

Codecov Report

Merging #156 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #156   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          34     34           
  Lines         694    696    +2     
  Branches       85     85           
=====================================
+ Hits          694    696    +2

Flag	Coverage Δ
#builder	100% <100%> (ø)	⬆️
#runtime	100% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
loader/sky-processor/index.js	100% <100%> (ø)	⬆️

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