textproc/docproj: Add new dependency

For an upcoming feature in the doc project we are adding image
facilities like drawing dynamic images/graphs with gnuplot add some new
dependency. A sample review can be found here
at https://reviews.freebsd.org/D36982.

In the future we can add more options but for add gnuplot-lite and
rubygem-asciidoctor-diagram for the image generation.

Approved by: doceng (fernape)
Differential Revision: https://reviews.freebsd.org/D36982