Add "line" style and "text_pos" argument to annotation_scale()

[ethanplunkett]

Update annotation_scale():

• Add text_pos argument to position the scale bar length text centered above the bar.
• Add "line" option to the style argument.
• All existing behavior of the function is unchanged by new arguments and options.

[ethanplunkett]

Here's an example of the new "line" style scale bar:

 p + annotation_scale(style = "line", width_hint = .15, location = "br")