Highcharts: add support for lists in colors and series methods

`colors` and `series` methods now also accept a list of objects. It checks whether the argument is one unnamed list or named parameters. This makes it easier to add several series at once (providing them in a list), instead of using `sapply` on the `series` method. This is actually how it works in the ordinary Highcharts API.

For an example, see the last part of the `series` section on the following page: [http://reinholdsson.github.io/rcharts-highcharts-api-docs/](http://reinholdsson.github.io/rcharts-highcharts-api-docs/).

It should (hopefully) not break any existing code; I have tested with the examples in the package, as well as a couple of gists.