This PR fixes document marker (--- and ...) detections since the current lexer implementation doesn't check whether those markers is at the beginning of a line or not, which makes it impossible to distinguish the following tokens with different meanings in YAML grammer.

--- # this is a directives end marker
  --- # this is a plain scalar

Also, the parser now emit an error if a document marker is found in a flow collection since the marker terminates the parsing and the corresponding ending character of the flow collection is regarded as missing:

[   # the corresponding `]` character is not found in this document since
--- # this marker terminates the parsing of the document.
]

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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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