cameo: migrated Excellon reader from drl2gp

- Makefile (OBJS): added excellon.o
- README: added brief description of Excellon input
- excellon.h, excellon.c: adaptation of KiCad drill file reader from
  ../drl2gp/drl2gp.c
- lang.l, lang.y: added command "excellon" to read KiCad drill files
- gerber.h: removed stray newline

cameo/lang.l

@@ -51,6 +51,8 @@ NUM	-?[0-9]+\.?[0-9]*
 				  return TOK_GERBER; }
 <INITIAL>gnuplot		{ file_name_follows = 1;
 				  return TOK_GNUPLOT; }
+<INITIAL>excellon		{ BEGIN(FILENAME);
+				  return TOK_EXCELLON; }
 <INITIAL>write			{ BEGIN(FILENAME);
 				  return TOK_WRITE; }