JSON for vectors of traces.

[EricForgy]

My previous PR ( #38 ) allowed for string interpolation of basic PlotlyJS types including GenericTrace. In addition to those, we also need vectors of generic traces. This PR completes that.

I use this in Pages.jl for sending JS commands from Julia to a browser, e.g.

n = 10
trace1 = scatter(;x = 1:n, y = rand(n))
trace2 = scatter(;x = 1:n, y = rand(n))
data = [trace1, trace2]
layout = Layout(;title = "Create a new <div> and plot an array of traces",
    width = 600,
    height = 300
)
id = "Plot2"
add_div(id)
Pages.broadcast("script","""Plotly.newPlot("$(id)",$(data),$(layout));""")

Without this PR, I'd need to do this instead:

n = 10
trace1 = scatter(;x = 1:n, y = rand(n))
trace2 = scatter(;x = 1:n, y = rand(n))
data = [trace1, trace2]
layout = Layout(;title = "Create a new <div> and plot an array of traces",
    width = 600,
    height = 300
)
id = "Plot2"
add_div(id)
Pages.broadcast("script","""Plotly.newPlot("$(id)",[$(trace1),$(trace2)],$(layout));""") # Note the individual traces.

[sglyon]

Looks great to me, thanks!