Generate CSS string representing grid template areas.
data_frame_to_css_grid_template_areas(areas_dataframe)
data.frame of character representing grid areas
character
This is a helper function used in grid_template()
areas_dataframe <- rbind( c("header", "header", "header"), c("menu", "main", "right1"), c("menu", "main", "right2") )result == "'header header header' 'menu main right1' 'menu main right2'"