add support for partial updates in style(), fixes #1342

[cpsievert]

No description provided.

[jcheng5]

I don't think this is quite right. The plotly.js semantics are that whatever object is found at the property path, is completely replaced. Your semantics appear to be more like if there is a . in the property path, then do a merge. Here's an example that demonstrates the difference:

p <- plot_ly(x = 1:10, y = 1:10, symbol = I(15), marker = list(
  line = list(
    width = 5,
    color = "orange"
  )
))
p

# According to plotly.js semantics, this should revert the marker line
# back to its default color, but with this PR it's still orange
style(p, marker.line = list(width = 10))

[jcheng5]

^

[jcheng5]

So, this is fine but I think you could've also just replaced both of these lines (73 and 74) with:

trace[[path[1]]] <- trace_replace(trace[[path[1]]] %||% list(), path[-1], value)