Add individual parameters for pen settings

The current setup for the `pen` parameter involves passing the literal GMT string. I think that this is confusing and non-intuitive for individuals who haven't used GMT. I think that the PyGMT wrappers for GMT modules that take a `pen` parameter should have parameters such `pen_color`, `pen_width`, `pen_color`, which will then be inputs to a function that returns a GMT-formatted string that is passed to the GMT API.

**Are you willing to help implement and maintain this feature?** Yes! I have a pretty good idea of how this would work, but want to gauge community interest before creating a pull request.