Show dimensions on-the-fly when drawing rectangle

[dbermond]

Can you please add an option to display the dimensions inside the rectangle on-the-fly while it's being drawn? Just like Spectacle does.

This would allow the user to select a precise dimension while drawing, for example a precise region of 1280x720. This is useful to avoid selecting unwanted resolutions (like 1283x721) that won't fit for a particular need.