Fix TOML 1.0.0 compliance

[sdispater]

This PR fixes tricky and edge cases to ensure full compliance with TOML 1.0.0.

Most of the test cases come from https://github.com/iarna/toml-spec-tests which has a comprehensive test suite.

[codecov-commenter]

Codecov Report

Merging #102 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   91.91%   91.80%   -0.12%     
==========================================
  Files          10       11       +1     
  Lines        2276     2391     +115     
==========================================
+ Hits         2092     2195     +103     
- Misses        184      196      +12     

Impacted Files	Coverage Δ
tomlkit/exceptions.py	93.02% <0.00%> (-1.85%)	⬇️
tomlkit/container.py	86.46% <0.00%> (-1.52%)	⬇️
tomlkit/items.py	88.14% <0.00%> (-0.27%)	⬇️
tomlkit/__init__.py	100.00% <0.00%> (ø)
tomlkit/_utils.py	97.61% <0.00%> (+0.28%)	⬆️
tomlkit/parser.py	96.53% <0.00%> (+0.37%)	⬆️
tomlkit/source.py	95.45% <0.00%> (+0.90%)	⬆️
tomlkit/toml_char.py	94.28% <0.00%> (+2.85%)	⬆️

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