Description of the desired feature

There should be a way to plot a title on two lines instead of just one. Carried on from discussion at https://forum.generic-mapping-tools.org/t/how-to-plot-title-with-a-subtitle-on-a-newline/1121

Current state

Using newline \n doesn't work on GMT 6.1.1:

gmt basemap -R0/1/0/1 -JX1 -Baf+t"Title\nSubtitle" -png map

Desired state

The "subtitle" should be on a second line.

It might be nice to have slightly different font sizes for the main title and subtitle. But making the newline \n work will do for now 😄

Are you willing to help implement and maintain this feature? No