enhancement: Expand with default values

[gabrielcipriano]

resolves #279 (and resolves #250 (?))

Approach:

To capture the default values when expanding envs, we need to somehow have access to the previously resolved variables.

So, the get() function needs an auxiliar data structure (map[string]string) that will keep track of the resolved raw strings. In that way, we can take advange of os.Expand(s string, mapping func(string) string) to provide our own mapping function that returns the resolved string value for a given key.

To avoid passing this auxiliar data structure as a parameter down the function calls, I added it as an unexported field of the Options struct, let's say, using opts as if it were a ctx.

I'm not really sure if it's the best way of doing this, any suggestions will be more than appreciated!

edit: typo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (13ac655) 100.00% compared to head (e2f4a5a) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #285   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          464       477   +13     
=========================================
+ Hits           464       477   +13     

Files	Coverage Δ
env.go	100.00% <100.00%> (ø)

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[caarlos0]

Made a small note only.

Could you also note this new feature somewhere in the readme?

Thanks for the PR 🙏

[gabrielcipriano]

Made a small note only.

Could you also note this new feature somewhere in the readme?

Thanks for the PR 🙏

Sure!
I'll be updating the readme soon

[gabrielcipriano]

I added an example covering common cases (using envDefault, and envDefault being overrided)

[caarlos0]

amazing, thank you!