[chloggen] Introduce repo-level config file

[djaglowski]

This PR introduces an optional repo-level configuration file for the chloggen tool.

• Add a global --config flag that can be used to specify a config file.
• Removes the chloggen-directory flag, which is not used anywhere in our org. Instead, this value can be specified within the config file.
• Renames the ChloggenDir field to ChlogsDir, since it is no longer required that this directory be used for "meta" files, such as config and template.
• Ensures that "meta" files (config, template) are ignored when reading/deleting entries. This allows for these files to be placed in the chlogs dir, or not.
• Adds an example config file that can be copied into place when setting up a repo. In the future this can likely be handled by a chloggen init command.

[codecov]

Codecov Report

Patch coverage: 82.05% and project coverage change: +0.20 🎉

Comparison is base (d2e2f13) 61.88% compared to head (e93b556) 62.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
+ Coverage   61.88%   62.08%   +0.20%     
==========================================
  Files          48       48              
  Lines        2330     2345      +15     
==========================================
+ Hits         1442     1456      +14     
- Misses        739      741       +2     
+ Partials      149      148       -1     

Impacted Files	Coverage Δ
chloggen/cmd/root.go	64.86% <30.00%> (-16.39%)	⬇️
chloggen/cmd/new.go	78.78% <100.00%> (ø)
chloggen/cmd/validate.go	84.21% <100.00%> (ø)
chloggen/internal/chlog/entry.go	81.53% <100.00%> (+4.61%)	⬆️
chloggen/internal/config/config.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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