Print markdown code with correct backticks

This adds extra backticks and spaces around code fragments when printing
if the fragment contains backticks. This allows `Markdown.parse` to
properly re-parse text correctly, i.e.

    Markdown.plain(Markdown.parse("``` `x` ```")) == "``` `x` ```"

rather than

    Markdown.plain(Markdown.parse("``` `x` ```")) == "``x``"

base/markdown/render/plain.jl

@@ -105,7 +105,16 @@ plaininline(io::IO, md::Bold) = plaininline(io, "**", md.text, "**")
 
 plaininline(io::IO, md::Italic) = plaininline(io, "*", md.text, "*")
 
-plaininline(io::IO, md::Code) = print(io, "`", md.code, "`")
+function plaininline(io::IO, md::Code)
+ if contains(md.code, "`")
+ n = maximum(length(m) for m in matchall(r"(`+)", md.code))
+ s = "`"^((iseven(n) ? 1 : 2) + n)
+ print(io, s, Base.startswith(md.code, "`") ? " " : "")
+ print(io, md.code, endswith(md.code, "`") ? " " : "", s)
+ else
+ print(io, "`", md.code, "`")
+ end
+end
 
 plaininline(io::IO, br::LineBreak) = println(io)
 

test/markdown.jl

@@ -55,6 +55,11 @@ code_in_code = md"""
 @test code_in_code == MD(Code("```"))
 @test plain(code_in_code) == "````\n```\n````\n"
 
+let text = "Foo ```bar` ``baz`` ```\n",
+ md = Markdown.parse(text)
+ @test text == Markdown.plain(md)
+end
+
 @test md"A footnote [^foo]." == MD(Paragraph(["A footnote ", Footnote("foo", nothing), "."]))
 
 @test md"[^foo]: footnote" == MD(Paragraph([Footnote("foo", Any[" footnote"])]))