UPDATE July 21, 2018: Sketch v51 FINALLY fixes the truly awful default arrows that used to come with sketch! Future work on this plugin will likely cease given that there's now a built-in decent arrowhead that will only get better with each later release.
This plugin adds simple solid arrowheads to any line, sized and colored to match path style.
Double-click on .sketchplugin file.
Select one or multiple paths, then hit Ctrl-Shift-A to add auto-styled arrowheads to end. Menu allows adding arrowhead to beginning or both ends of paths instead.