Update boundary line tools to output a labeled mask

[nfahlgren]

Is your feature request related to a problem? Please describe.
The tools pcv.analyze.bound_horizontal and pcv.analyze.bound_vertical output illustrative images for debugging purposes. These are useful, especially when developing a workflow. But another useful output would be a labeled mask where the upper and lower or left and right parts of the object are labeled 1 and 2 respectively. This would allow someone to input the separate parts into other analyze functions to measure size/shape and color of the two parts (e.g. if one split roots and shoots using the line tools).

Describe the solution you'd like
It is a little complicated because to keep the function output structure the same we could just output the labeled mask instead of the illustrative image. But folks like to save the current image for debugging or figures.