This is Thomas Lübking's "Screen Snapping" KWin script, now for KDE Plasma 5. A couple of new problems that appeared in KDE5 are fixed: configuration dialog works again, and newly-placed windows will be properly snapped (if applicable).
Thomas's original description: "This overrides stock screen snapping and snaps to the decoration directly (unlike snap-to-deco which fixes the position after the actual snap and stopping the movement). In addition, vertical, horizontal, inner and outer snap distance can be configured independently."
To install manually: plasmapkg2 -i screenSnapping5.kwinscript -t kwinscript
Be sure to go to System Settings -> Window Management -> KWin Scripts, to enable (and configure if desired) after installing.