plotCorrelation scatterplot needs option to alter scales

[connorrogerson]

PlotCorrelation scatterplots are useful for checking correlation between replicates but the tool needs an option to define the maximum of an axis. I've had many plots where all the data are squished into the bottom left corner because the scales are way off. I'm also already using --skipZeros and --removeOutliers options.

[dpryan79]

I've implemented this in the develop branch for the 2.6.0 branch.