implement var substitution feature

[mikerott]

The README already implied that the feature this PR introduces was a thing. It was not, but after this PR, it is! Consider this PR a proposal for a feature, and because I did not add feature-specific function names to the SDK, this PR represents a breaking change and will require a major version update. That said, I expect that no one will actually be broken upon updating.

Below was my original review request and is now outdated.

All backward compatibility remains intact, so this will be a minor version update.

I have a specific review question. This PR, as it is, behaves such that if a substitution is desired, but implemented incorrectly, the result is to return nil, false (nil value, not found) rather than the unsubstituted value.

For example, imagine the env vars are:

A=aValue

and the YAML file is:

B: "${C}"

The SDK could behave such that "find B" returns nil, false or it returns "${C}", true. I think I prefer the former, as it's the earliest warning sign to the user of this lib that they're seeking a key (C) that does not exist.

[codecov]

Codecov Report

Merging #30 (9b015aa) into master (604a53e) will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   91.27%   91.60%   +0.32%     
==========================================
  Files           6        6              
  Lines         871      905      +34     
==========================================
+ Hits          795      829      +34     
  Misses         62       62              
  Partials       14       14              

Impacted Files	Coverage Δ
source.go	76.81% <100.00%> (+7.58%)	⬆️

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