Support multiple config file types

[rappizs]

Fixes #35

Dev notes

• Added support for Yaml and Toml
• Integrated (basically copied with some refactor) part of Viper's encoding package: https://github.com/spf13/viper/tree/master/internal/encoding

Why copied the encoding package instead of using Viper's Write methods? Unfortunately those methods have some problems and they are not reliable (A Viper contributor told me not to use them). They will be removed from Viper and after some discussion I decided to reuse their code. They also told me that the encoding package will be a separate project eventually, so as soon as that happens we can replace our encoding package with that.

[codecov]

Codecov Report

Merging #172 (1728445) into main (ee1bd0b) will increase coverage by 4.03%.
The diff coverage is 24.63%.

❗ Current head 1728445 differs from pull request most recent head e368b40. Consider uploading reports for the commit e368b40 to get more accurate results

@@           Coverage Diff            @@
##            main    #172      +/-   ##
========================================
+ Coverage   0.00%   4.03%   +4.03%     
========================================
  Files          1       5       +4     
  Lines         67     421     +354     
========================================
+ Hits           0      17      +17     
- Misses        67     401     +334     
- Partials       0       3       +3     

Files Changed	Coverage Δ
internal/config/config.go	2.38% <15.68%> (ø)
internal/encoding/json/codec.go	50.00% <50.00%> (ø)
internal/encoding/toml/codec.go	50.00% <50.00%> (ø)
internal/encoding/yaml/codec.go	50.00% <50.00%> (ø)

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[justaugustus]

branch conflict needs to be resolved