add pushing of subfields

[hhaensel]

With this PR it is possible to push to subfields, e.g.

push!(model, Symbol("plot.data") => Plot(scatter(y = 1:10)).data)

or

push!(model, :var"plot.data" => Plot(scatter(y = 1:10)).data)

This is currently only implemented on the js side.
We could implement it on the Julia side as well to support

push!(model, :var"plot.data")

We could also support array indexing

@essenciary , @protozoo , @PGimenez
Happy to hear, what you think.

[hhaensel]

Array indexing works already without adaptation:

hh = {data: 'test', layout: {nest: ['me', 'you']}}
getField(hh, 'layout.nest.1'.split('.'))
// 'you'