Delay opening of config file to actual command execution

[jwodder]

This makes it possible to do tinuous fetch --help without a config file present.

[codecov-commenter]

Codecov Report

Merging #116 (269c43f) into master (95da3a3) will increase coverage by 27.83%.
The diff coverage is 46.15%.

@@             Coverage Diff             @@
##           master     #116       +/-   ##
===========================================
+ Coverage   44.45%   72.29%   +27.83%     
===========================================
  Files           9       10        +1     
  Lines         893      906       +13     
  Branches      134      134               
===========================================
+ Hits          397      655      +258     
+ Misses        495      198      -297     
- Partials        1       53       +52     

Impacted Files	Coverage Δ
src/tinuous/__main__.py	53.23% <46.15%> (+53.23%)	⬆️
src/tinuous/_version.py	100.00% <0.00%> (ø)
src/tinuous/util.py	86.36% <0.00%> (+6.81%)	⬆️
src/tinuous/config.py	92.63% <0.00%> (+18.94%)	⬆️
src/tinuous/github.py	51.00% <0.00%> (+20.50%)	⬆️
src/tinuous/base.py	87.30% <0.00%> (+25.39%)	⬆️
src/tinuous/travis.py	69.09% <0.00%> (+40.90%)	⬆️
src/tinuous/appveyor.py	77.38% <0.00%> (+45.23%)	⬆️

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[yarikoptic]

I haven't tried, but the motivation sounds legit ;) thank you!