More flexible tooling for arbitrary line additions

[j-andrews7]

As mentioned in #17, this breaks out the line additions to their own functions in utils_plot_mods.R. Biggest changes are the addition of arbitrary ablines, linewidth and opacity parameters, doc additions, and a bit of code refactoring.

It also explicitly mentions that any of the line parameters can accept a vector of the same length of the lines being added to style each individually, e.g.:

scatterPlot(
         example_df, x.by = "PC1", y.by = "PC2",
         color.by = "gene1",
         size = 3, add.xline = c(-1, 0, 1), xline.color = c("blue", "red", "green"), 
         xline.linetype = c("dotdash", "dashed", "solid"), xline.opacity = 0.7, xline.linewidth = c(1, 2, 3),
         add.yline = c(-2, 2), yline.color = "magenta", 
         add.abline = c(0, 1), abline.slope = c(1, -0.5), abline.linewidth = 1,
         abline.color = c("skyblue", "darkred"), split.by = "groups")

Think this is ready for review, tests pass.