Added functionality to dumbplot

Added functionality to dumbplot to do a few other things I found helpful:

. Treat x-coordinates as categorical data, plot them in the order that they come in.  Useful for things like dates in ISO format that you want to quickly plot but don't want to convert to a numeric value to be compatible with scatter plots.
. Use a barchart rather than just a scatterplot (also assumes categorical data in x-coordinate).
. Set ascii plot width to full width of your terminal, and plot height to half of terminal height.  Also allows custom column and lines (width / vertical height) command line parameters.

Note that it also has a dependency on the `nl' command.