Initial implementation of config stack

[funbringer]

The main idea is to allow user to have several global configs stored in some stack. It's also possible to temporarily change global config using context manager called scoped_config.

• Renamed TestgresConfig class to GlobalConfig;
• Added object TestgresConfig for compatibility (== testgres_config).
• Added functions: push_config(), pop_config(), scoped_config().
• Small refactoring in relevant modules.

[codecov-io]

Codecov Report

Merging #34 into master will decrease coverage by 0.02%.
The diff coverage is 97.74%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   97.26%   97.24%   -0.03%     
==========================================
  Files          13       13              
  Lines        1098     1162      +64     
==========================================
+ Hits         1068     1130      +62     
- Misses         30       32       +2

Impacted Files	Coverage Δ
testgres/consts.py	100% <ø> (ø)	⬆️
testgres/node.py	97.55% <100%> (-0.01%)	⬇️
testgres/exceptions.py	100% <100%> (ø)	⬆️
testgres/cache.py	90.9% <100%> (+1.25%)	⬆️
testgres/__init__.py	100% <100%> (ø)	⬆️
testgres/utils.py	97.82% <100%> (+0.01%)	⬆️
testgres/config.py	97.01% <96.61%> (-2.99%)	⬇️
tests/test_simple.py	99.72% <97.56%> (-0.28%)	⬇️

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[funbringer]

@stalkerg maybe you could take a look at the code and share your opinion.

[stalkerg]

In my opinion, the code is good and it's can help to users in this case but I am not sure because all this user can do in his own test (get global config and return back after a while).
Right now I don't know your cases very well and if it really can help this means everything fine but we should be careful, we can make fat tool.