broken text/latex output for markdown horizontal rule

[stevengj]

Markdown.MD objects claim to support text/latex output, but there is a MethodError for HorizontalRule elements (from JuliaLang/IJulia.jl#776):

julia> using Markdown

julia> m = Markdown.parse("---")
  ────────────────────────────────────────────────────────────────────────────

julia> showable("text/latex", m)
true

julia> show(stdout, "text/latex", m)
ERROR: MethodError: no method matching latex(::Base.TTY, ::Markdown.HorizontalRule)
Closest candidates are:
  latex(::IO, ::Markdown.Table) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/GitHub/table.jl:154
  latex(::IO, ::Markdown.LaTeX) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/IPython/IPython.jl:28
  latex(::IO, ::Markdown.MD) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/render/latex.jl:19
  ...
Stacktrace:
 [1] latex(::Base.TTY, ::Array{Any,1}) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/render/latex.jl:23
 [2] latex at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/render/latex.jl:19 [inlined]
 [3] show(::Base.TTY, ::MIME{Symbol("text/latex")}, ::Markdown.MD) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Markdown/src/render/latex.jl:172
 [4] show(::Base.TTY, ::String, ::Markdown.MD) at ./multimedia.jl:79
 [5] top-level scope at none:0