Add the -t option for psevents

[Esteban82]

In this option I add the -t option for psevents module. This PR refers to the first issue in #8775.

I tried it with these commands:

gmt makecpt -Cred,green,blue -T0,70,300,10000 > q.cpt
gmt events -Rg -JG200/5/6i -Baf @quakes_2018.txt -SE- -Cq.cpt  --TIME_UNIT=d -T2018-06-01T  -t50 -png prueba

I got this figure

If it is ok, later I will modify the docs.

[Esteban82]

Thanks Remko. I've just add the explanation in the docs.