add simple runtime configuration

[leifwalsh]

This allows us to disable any request types we don't want nsncd to handle.

fixes runtime configuration #42

[codecov-commenter]

Codecov Report

Base: 30.04% // Head: 41.60% // Increases project coverage by +11.55% 🎉

Coverage data is based on head (f9736cf) compared to base (82dd540).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #51       +/-   ##
===========================================
+ Coverage   30.04%   41.60%   +11.55%     
===========================================
  Files           5        6        +1     
  Lines         213      274       +61     
===========================================
+ Hits           64      114       +50     
- Misses        149      160       +11     

Impacted Files	Coverage Δ
src/main.rs	0.00% <0.00%> (ø)
src/protocol.rs	40.90% <ø> (ø)
src/handlers.rs	33.33% <50.00%> (-2.23%)	⬇️
src/config.rs	85.96% <85.96%> (ø)

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[leifwalsh]

Ok, I tried downgrading all the way to clap v2, whose API is miserable compared to modern clap, and it still doesn't compile on debian 10. I think I'm giving up on this path for now.