Gnuplot added

[Gnuplot][homepage] is widely used on GitHub (you can find several examples using a search on its extensions, for example [here][s1], [here][s2], [here][s3] or [here][s4]). Pygments has a [lexer][] for it.

[homepage]: http://www.gnuplot.info/
[s1]: https://github.com/search?q=extension%3Agp+a
[s2]: https://github.com/search?q=extension%3Agnu+a
[s3]: https://github.com/search?q=extension%3Agnuplot+a
[s4]: https://github.com/search?q=extension%3Aplt+a
[lexer]: http://pygments.org/docs/lexers/#pygments.lexers.other.GnuplotLexer