Improved line removed markers

Based on @felixkiss' suggestion from #20. This is tweaked a bit to make it look
reasonable when the gutter is wide. I also use the same technique when icons
are enabled.
Use triangle for non-icon mode

Also changes icon mode back to using a dash instead of a border, but positions
it in between lines. This looks the same but seemed less of a hack.

Test plan:
Switched between both versions in a 90 line file and they look good.
Merge pull request #32 from dmnd/better-line-removed

Improve line removed markers / removed markers are confusing