WIP: Initial work to add modifiers to Cargo.toml and env variable options

[deg4uss3r]

This PR adds the ability to use modifiers in Cargo.toml and Environment variable overrides.

resolves: #118

[gdesmott]

Thanks for your contribution.

I have no objection adding such feature but would like a bit more context as I never used those kinds of modifiers.

Adding @xclaesse and @nirbheek in case they know more about modifiers and have some input.

Once we agree on the API we'll need it properly documented in the general documentation at the top of lib.rs.

[gdesmott]

I'm not familiar with +whole-archive and modifiers in general. If they are used only during static linking maybe they should be called static_modifiers instead?

What other modifiers could be used there for example?

[deg4uss3r]

Thank you so much for taking a look so quickly and for pulling in some more people, I wouldn't consider myself a deep expert either!

In rustc documentation the three specified are [+|-] whole-archive, verbatim, and bundle. This is where I wouldn't consider myself a deep expert and I'm not sure if more are supported and these 3 are just the documented ones because they have defaults.

The terminology I used here matches rustc and build script documentation. However, I am happy to rename if that's the consensus.

[gdesmott]

I think it makes sense to use the same terminology as rustc then. We should also link this page in our documentation of this feature.

[codecov]

Codecov Report

Attention: Patch coverage is 96.31902% with 6 lines in your changes missing coverage. Please review.

Project coverage is 91.30%. Comparing base (38acdbc) to head (dfff668).

Files with missing lines	Patch %	Lines
src/lib.rs	90.56%	5 Missing ⚠️
src/test.rs	98.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   90.91%   91.30%   +0.38%     
==========================================
  Files           3        3              
  Lines        2257     2403     +146     
==========================================
+ Hits         2052     2194     +142     
- Misses        205      209       +4     

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