Add support for horizontal and vertical lines from a single value in gmt plot

[Esteban82]

This PR adds support for plotting horizontal and vertical lines from a single coordinate value in gmt plot.

Two new symbols are introduced:

-SY plots a horizontal line spanning the full plot width.
-SX plots a vertical line spanning the full plot height.

Example script:

echo 1 > line.txt
echo 3 >> line.txt

gmt begin linea png
    gmt basemap -R0/4/0/4 -Baf -Jx1c
    gmt plot line.txt -SX -Wblue 
    gmt plot line.txt -SY -Wred
    echo 2 | gmt plot -SY -Wgreen,-
gmt end 

At the moment, the implementation only supports Cartesian coordinates.

The long-term goal is to extend the functionality to geographic and polar coordinate systems, similar to the implementation available in PyGMT to plot parallels and meridians, or arcs and straight lines along radius at a specified azimuth. It may also be possible to add analogous functionality to plot3d in the future, including a -SZ option for drawing reference lines along the Z axis.

Question:

Are the proposed symbol codes -SX and -SY acceptable, or would you prefer different letters?