plot_cover: Plot Cover Extent
In crumplecup/riparian: Analyzing the Extent of Riparian Cover

Prints a graph of cover extent.

plot_cover(
  csv,
  title = "cover_change.png",
  heading = " ",
  leg_pos = "topleft",
  y_lab = "Proportion of Samples at or below X% Cover"
)

`csv`	is a csv with cols (id, year, type, results(1:50))
`title`	is the character vector to name the output file (png)
`heading`	is the character vector used as the main title of the plot
`leg_pos`	is the legend position argument (character)
`y_lab`	is the label for the y axis (character)