Add `sel_plot` function

[bschneidr]

This function would ultimately have a result similar to sel_image() in that the resultant HTML/Javascript would cycle through base 64 images or image file references. However, when the user is working in RMarkdown, they would feed plot objects to the function, rather than feeding image file references.

plot_a <- qplot(x = Sepal.Length, y = Sepal.Width, data = iris)
plot_b <- qplot(x = mpg, y = wt, data = mtcars)
sel_plot(sel_id = "datasets",
         options = list(a = plot_a, b = plot_b),
         embed = TRUE)

As with sel_image(), the user can choose whether to have base-64 images or linked image files.

[bschneidr]

Will want to make sure that this works with a variety of plot objects, such as ggplot, base plots, gtable, etc.

[bschneidr]

Will also want to make sure that the function is aware of relevant knitr global and chunk options (e.g. fig.width) when generating images.

[bschneidr]

Still need to make the function work with non-ggplot2 plot objects.