Fix for hidden crosshairs + new option + new feature

[tr8dr]

Fix
Crosshairs were hidden beneath any shading in a graph. This was due to the crosshair canvas being below the graph canvas. This was solved by changing the z-index, position type, and allowing events to pass through to lower canvas layers.

New Option
Also added a color option to dyCrosshairs(), to allow users to change the color of the crosshair.

New Feature
Added dyUpdate(session, id, data). This allows one to update an existing dygraph from shiny without rerendering the widget in the DOM. Added functionality in R, JS, and documentation.

[przmv]

LGTM!

Thanks, @tr8dr! Could you please also update the docs and the NEWS file?

[tr8dr]

I have adjusted the Rmd and html docs for the new functionality. I did not find a "news" file anywhere in the distribution however. Let me know if there is something additional to be updated here.

[tr8dr]

Let me know if you need additional changes. Have added a new feature to push data updates to an existing dygraph in the browser from R.

[przmv]

Hey, @tr8dr, sorry for the delay. Could you please submit separate pull request for the independent changes? It would make the reviewing process quicker and easier. Thanks!

[tr8dr]

hmm, if you need it this way, I would have to unwind the second set of changes and wait for you to integrate the first set and then do a pull on that integrated set and put those changes back in. Not sure if there is a magical way to do this in github.

[przmv]

@tr8dr FYI: https://glebbahmutov.com/blog/splitting-pull-request/

[tr8dr]

Thanks, I will take a look at the above on the weekend ...