feat(parser): support front-matter in YAML format

[xcoulon]

Front-matter tokens must be ---.
Front-matter content is parsed using the gopkg.in/yaml.v2
library: no need to recreate the YAML grammar here !

Signed-off-by: Xavier Coulon [email protected]

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.19%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage    65.8%   65.99%   +0.19%     
==========================================
  Files          19       19              
  Lines        2129     2194      +65     
==========================================
+ Hits         1401     1448      +47     
- Misses        621      637      +16     
- Partials      107      109       +2

Impacted Files	Coverage Δ
parser/asciidoc_parser.go	57.75% <ø> (+0.82%)	⬆️
types/document_attributes.go	100% <100%> (ø)	⬆️
renderer/html5/document_attribute.go	88.88% <60%> (-11.12%)	⬇️
types/grammar_types.go	67.46% <64.7%> (-0.23%)	⬇️

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