Generalize import of graphical objects

It would be nice to have a general mechanism for adding graphical objects (like ggplot, highcharts, plotly etc.).

Requirements

1. a single template (inst/templatest/vis_object.Rmd) that makes use of two additional placeholders ({{ package }} and {{ class }}).
2. the add_object function, renamed to add_vis_object in R/vis_objects.R (needs modification to work with the new inst/templatest/vis_object.Rmd!).
3. the newly generated method functions in R/vis_objects.R (hint: they can be shortened to one-liners).