Make a harpoon arrowhead

## Arguments

- point_angle
angle of harpoon point

- barb_angle
angle of harpoon barb

- degrees
if TRUE, angles are in degrees instead of radians

- rotate
rotation angle in radians

- rescale
a single value or 2-length vector for scaling in x and y

- nudge
a single value or 2-length vector for nudging in x and y

- transformations
a vector of transformation functions

- plot
plot arrowhead if TRUE