Provide a verbatim mode

[t-bltg]

Xref JuliaPlots/Plots.jl#4514.

julia> using GR
julia> plot(1:2, title="\$")
ERROR: syntax error
ERROR: syntax error

It would be useful to provide a verbatim mode (i.e. don't try to interpret title, labels, ... as pseudo-latex commands).

Also, why not support LaTeX inline math mode, instead of plain TeX ?

julia> using GR
julia> plot(1:2, title=raw"\(\sqrt{2}\)")