Defines a major mode for editing gnuplot scripts.  I wanted to keep
it simpler than other modes -- just syntax highlighting, indentation,
and a command to plot the file.

Some of this code is adapted from a more full-featured version by
Bruce Ravel (available here https://github.com/bruceravel/gnuplot-mode;
GPLv2).

Thanks to everyone, including Christopher Gilbreth and Ralph Möritz,
for sending suggestions, improvements, and fixes.