PT-6909 Extracted spec loading to config and added some tests

[vgarvardt]

First piece extracted from bigger brother #110

In this PR:

• improved error reporting
• moved config loading to config package and covered it with basic tests
• fixed init command as generated toml file has invalid structure - go sruct embedding for some reasons works differently for writing toml and reading it

[codecov]

Codecov Report

Merging #111 into master will increase coverage by 3.1%.
The diff coverage is 51.85%.

@@            Coverage Diff            @@
##           master     #111     +/-   ##
=========================================
+ Coverage   32.09%   35.19%   +3.1%     
=========================================
  Files           5        6      +1     
  Lines         215      233     +18     
=========================================
+ Hits           69       82     +13     
- Misses        142      145      +3     
- Partials        4        6      +2

Impacted Files	Coverage Δ
pkg/dumper/query/dumper.go	0% <0%> (ø)	⬆️
pkg/anonymiser/anonymiser.go	69.76% <100%> (ø)	⬆️
pkg/config/config.go	72.22% <68.75%> (ø)

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[boekkooi-fresh]

👍 LGTM

[samacoff]

+1