Indicate unused blocks in master index

It gives a better idea of where stuff can be moved/added to.

blk/001

@@ -1,12 +1,15 @@
 MASTER INDEX
 
+5-99 unused
 100 Block editor
 120 Visual Editor             150 Extra words
+160-199 unused
 200 Z80 assembler             260 Cross compilation
 280 Z80 boot code             350 Core words
-410 PS/2 keyboard subsystem
+410 PS/2 keyboard subsystem   420-489 unused
 490 TRS-80 Recipe             520 Fonts
 550 TI-84+ Recipe             580 RC2014 Recipe
 620 Sega Master System Recipe
-650 AVR assembler             730 8086 assembler
+650 AVR assembler             690-729 unused
+730 8086 assembler
 800 8086 boot code            830 PC/AT recipe