Allow altering of selection after releasing mouse button

[PucklaJ]

I don't know if this is possible with the way slurp is currently implemented, but it would be nice if you could add a flag that lets the user alter the selection after the initial selection has been made and the mouse button has been released similar to how flameshot does it.
UI wise this could be implemented by adding small circles or rectangles to the edges of the selection box and when dragging these the selection gets altered. When pressing the return key or some other key which can be configured the selection is finished and output to stdout.

[m-bartlett]

I addressed this partially in #138, it only supports re-selecting the entire rectangle rather than modifying individual corners. I wanted to start with implementing the basic feature but it probably could be extended to allow modifying the existing selection.

[Photon89]

That would be a great feature! I have just proposed to use Slurp in Shutter, as we are quite desperate regarding Wayland support... Our selection tool (which only works in X11) supports modifying the selection as well, so it'd be great if Slurp could also support this!