Figure.meca: Allow specifying the label angle, font, justification, and offset

[maxrjones]

Description of the desired feature

Based on this forum post, it would be useful to add support for customizing the label characteristics for meca, which is done using -S in gmt meca. Relates to #1082, in order to make the PyGMT implementation more pythonic than gmt meca -S.

Are you willing to help implement and maintain this feature? Not likely that I will find time to implement

[weiji14]

Note that there was a previous attempt to handle the text label specification. See @tjnewton's implementation at 2fe87af, but this was reverted by @liamtoney at ff164e6 because we didn't support GMT_Put_Strings at that time (full explanation at #516 (comment).

I'll try and resurrect that old bit of code, and put it in #1613. Or, I could implement the text label and angle/font/justification specification in a separate Pull Request after #1613.