Enable MathJax functionality (#11)

* Enable MathJax functionality

Allow LaTeX to appear in images produced using savefig().

* Make MathJax configurable

src/PlotlyKaleido.jl

@@ -17,13 +17,18 @@ end
 
 const P = Pipes()
 
+const _mathjax_url_path = "https://cdnjs.cloudflare.com/ajax/libs/mathjax"
+const _mathjax_last_version = v"2.7.9"
+
 kill_kaleido() = is_running() && kill(P.proc)
 
 is_running() = isdefined(P, :proc) && isopen(P.stdin) && process_running(P.proc)
 
 restart(;kwargs...) = (kill_kaleido(); sleep(0.1); start(;kwargs...))
 
-function start(;plotly_version = missing, kwargs...)
+function start(;plotly_version = missing, 
+                mathjax = missing, mathjax_version::VersionNumber = _mathjax_last_version, 
+                kwargs...)
     is_running() && return
     cmd = joinpath(Kaleido_jll.artifact_dir, "kaleido" * (Sys.iswindows() ? ".cmd" : ""))
     basic_cmds = [cmd, "plotly"]
@@ -39,6 +44,21 @@ function start(;plotly_version = missing, kwargs...)
             kwargs...
         )
     end
+    if !(mathjax === missing)
+        if mathjax_version > v"2.7.9"
+            error("The given mathjax version $(mathjax_version) is greater than the last supported version of $(_mathjax_last_version).")
+        end
+        if mathjax isa Bool && mathjax
+            push!(chromium_flags, "--mathjax=$(_mathjax_url_path)/$(mathjax_version)/MathJax.js")
+        elseif mathjax isa String  
+            # We expect the keyword argument to be a valid URL or similar, else error "Kaleido startup failed with code 1".
+            push!(chromium_flags, "--mathjax=$(mathjax)")
+        else
+            @warn """The value of the provided argument 
+                    mathjax=$(mathjax) 
+                  is neither a Bool nor a String and has been ignored."""
+        end
+    end
     # Taken inspiration from https://github.com/plotly/Kaleido/blob/3b590b563385567f257db8ff27adae1adf77821f/repos/kaleido/py/kaleido/scopes/base.py#L116-L141
     user_flags = String[]
     for (k,v) in pairs(extra_flags)