Add mdconvert dispatch on Symbol/Expr
Add mdconvert dispatch on Symbol/Expr

The parsed Markdown can also contain symbols and expressions. We'll
just convert those to string when we encounter them.

MichaelHatherly/Lapidary.jl,mortenpi/Documenter.jl,JuliaDocs/Documenter.jl,MichaelHatherly/Documenter.jl