Feature request: allow side-by-side tabset chunk options to work in other output formats.

[Robinlovelace]

Following on from #2053 I wonder if in static formats like Markdown, PDF and Word the tabset solution could display the tabset headings as a group and only show the first, allowing the author to add a comment like "See online version for alternatives". MD output shown.

Source: Twitter

[IndrajeetPatil]

This will be especially great for github-flavored markdown format where you often need to display context-dependent code outputs or solutions (e.g.)!

Trying the tabset featuer with gfm, the issue seems to be that ## {.tabset} in .Rmd is rendered to ## in .md.