params.Frame: Add subtitle support

[seisman]

Address #4588

Example

import pygmt
from pygmt.params import Axis, Frame

fig = pygmt.Figure()
fig.basemap(
    region=[0, 20, 0, 3], 
    projection="M10c", 
    frame=Frame(
        axes="WSEN", 
        title="Title", 
        subtitle="SubTitle", 
        axis=Axis(annot=True)
    )
)
fig.show()