initial implementation of view-in-pane for R Markdown documents
persist scroll position across renders in pane
improve scroll sync between panes
maximize viewer on initial document render
show viewer preference checked/unchecked in menu
support scroll preservation in view frame for Shiny documents
cap retries on scroll adjustment
improve Shiny document switch between pane/window
always show presentations in a window (not the viewer pane)
make UIPref menu item selector more generic
shut down R Markdown render when Shiny doc running in viewer is cleared
don't show viewer type picker for Shiny presentations
clean warnings, comments, indentation, etc.
don't show pane switcher if output format is not known to be HTML
preserve scroll positions for Shiny documents in desktop mode
Merge pull request #130 from rstudio/feature/rmd-viewer-pane

Support for R Markdown documents in the Viewer pane