Support a catch all host which (almost) always matches the DNS request host

[bridgeythegeek]

This PR adds support for a [[zones]] entry for host with name __CATCH_ALL which will always match a host name when tested.

• The __CATCH_ALL answer is only provided if an exact match hasn't already been found in [[zones]] entries above the __CATCH_ALL entry in the toml file.
• [[zones]] entries below the __CATCH_ALL entry may also match as well as the __CATCH_ALL.

Resolves #21.

[codecov]

Codecov Report

Merging #22 (de88464) into main (a99dfe9) will decrease coverage by 2.53%.
The diff coverage is 57.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   97.54%   95.02%   -2.53%     
==========================================
  Files           4        5       +1     
  Lines         204      221      +17     
  Branches       37       42       +5     
==========================================
+ Hits          199      210      +11     
- Misses          3        7       +4     
- Partials        2        4       +2     

Files	Coverage Δ
dnserver/main.py	94.44% <57.14%> (-4.68%)	⬇️

... and 1 file with indirect coverage changes

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