Escaping pipes in the code of table elements

[dearblue]

ref. https://github.github.com/gfm/#example-200

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The "yard" called by "mrbdoc" has several libraries available to parse Markdown.
Note: The preferred library can be changed with mrbdoc -M <gem-name>.

https://github.com/lsegal/yard/blob/v0.9.28/lib/yard/templates/helpers/markup_helper.rb#L24-L34
https://github.com/lsegal/yard/blob/v0.9.28/lib/yard/templates/helpers/markup_helper.rb#L87-L116

Ruby gem name	Support table?	Can pipes in codes inside tables be escapable?	Note
redcarpet	yes	no
rdiscount	yes	yes
kramdown	yes	yes, but escaped characters remain
bluecloth	no
maruku	yes	no	HTML tags are not allowed in Markdown
rpeg-markdown	?	?	require "rpeg-markdown" fails; archived repository
rdoc	no		Maybe support in the next rdoc? ruby/rdoc#931
commonmarker	no		Maybe support in the next yard? lsegal/yard#1443

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In "doxygen", the | directive in mrb_get_args() in include/mruby.h is fine, but in opcode.md the escaping for pipe remains.