Support multiple viewer options and dispatch onLoad

[michaelquinn32]

Html might be a more universal format for printing skim results. We should consider a path based around Rstudio viewer() that generates html for the user. We would need to decide on a path for spark plots:

• One approach in kableExtra https://cran.r-project.org/web/packages/kableExtra/vignettes/awesome_table_in_html.html#Insert_Images_into_Columns
• Another approach in sparkline https://github.com/htmlwidgets/sparkline

We could also assign the appropriate skim function onLoad/ onAttach:

• If you have the viewer function, you get the html skim output
• If you you have standard utf8 support, use standard skim
• Otherwise, skim_no_plots

[elinw]

I really like the idea of having the option for the RStudio viewer; kableExtra already does html although it doesn't render quite right. But at least that might be tweaking.