Preparation for next major version

[hukkin]

• Set dateformat to a constant
• Start graylog handler last so we get the warning logs from it in file/stdout
• Make loggo.loggo module private so the only public things/import paths are the ones in loggo.__init__: loggo.Loggo and loggo.__version__. This allows us to not prefix everything with _ in loggo.loggo module
• Make almost all data attributes of Loggo private

[codecov-io]

Codecov Report

Merging #130 into master will decrease coverage by 0.05%. The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   95.51%   95.45%   -0.06%     
==========================================
  Files           2        2              
  Lines         312      308       -4     
==========================================
- Hits          298      294       -4     
  Misses         14       14

Impacted Files	Coverage Δ
loggo/__init__.py	100% <100%> (ø)	:arrow_up:
loggo/_loggo.py	95.42% <93.61%> (ø)

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