Added '.drill' and '.drl' extensions for Excellon drill files.  These are used by KiCAD-generated files.
Added '.drill' and '.drl' extensions for Excellon drill files.  These are used by KiCAD-generated files.

jihyun-kim/heekscnc,jihyun-kim/heekscnc,asmuelle/heekscnc,satovey/heekscnc,satovey/heekscnc,satovey/heekscnc,asmuelle/heekscnc,satovey/heekscnc,jihyun-kim/heekscnc,asmuelle/heekscnc,asmuelle/heekscnc,jihyun-kim/heekscnc