Allow setting global hooks to modify Easy object

[habemus-papadum]

Provide a robust way to customize system download behavior (more robust
in comparison to #207)

Todo

• Scan for typos, code style
• Add unit tests
• Add documentation
• Create sample usage that sets global
  - Timeout options
  - CA paths
  - rewrite urls

[codecov]

Codecov Report

Merging #209 (57f08f1) into master (0098e40) will increase coverage by 0.34%.
The diff coverage is 100.00%.

❗ Current head 57f08f1 differs from pull request most recent head a30270e. Consider uploading reports for the commit a30270e to get more accurate results

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   90.26%   90.60%   +0.34%     
==========================================
  Files           5        5              
  Lines         575      596      +21     
==========================================
+ Hits          519      540      +21     
  Misses         56       56              

Impacted Files	Coverage Δ
src/Downloads.jl	87.87% <100.00%> (+2.29%)	⬆️

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[habemus-papadum]

Functionality and unit tests complete. If someone could have an initial glance, that would be much appreciated. Still needed: documentation

[habemus-papadum]

added documentation

[habemus-papadum]

I've rebased this PR. It should provide a more robust/composable solution to configuring systemwide download behavior. Any comments regarding the API would be welcome

[DilumAluthge]

@habemus-papadum Are you still interested in moving this PR forward? If so, can you rebase on the latest master, and fix the merge conflicts? I will then try to recruit someone to review this PR.