Create outline with markers

[olivroy]

• Investigate markers https://github.com/rstudio/rstudio/issues?q=is%3Aissue+is%3Aopen+marker, see if not too many issues come up with the next release. Seems that inline markup may show bizzare.
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[olivroy]

The markers argument can contains either a list of marker lists or a data frame with the appropriate marker columns. The fields in a marker are as follows (all are required):

Variable	Descr
type	The marker type ("error", "warning", "info", "style", or "usage").
file	The path to the associated source file.
line	The line number for the associated marker.
column	The column number for the associated marker.
message	A message associated with the marker at this location.