Support option of draw_gridlines

I see the gridlines are turned on by default. It's better and easier to add the parameter to control it:

def (..., draw_gridlines=True):

....
if draw_gridlines:
    ax.gridlines(draw_labels=True, linewidth=2, color='gray', alpha=0.5, linestyle='--')  # cartopy>=0.18.0 only

This is useful for users to control the gridlines on their own.

By the way, it seems I can't fork the repo to create a PR.