Add a command to manually mark data as code (domanual).

Useful for jumps based on Direct Page.

Hi! First of all, I wanted to thank you for this disassembler, I find it highly useful. For a project I'm currently working on, the disassembler misses some jumps based on variables in the Direct Page. I have added a command which will manually mark such sections as code based on the environment from which the code is jumped.

As always, this will not be perfect (though uncommon, consider the case where a piece of code can be reached from multiple Direct Page variables used as direct indirect operands), and will not find dead code. Additionally, all my additions do is annotate the code and add a label. Some future work would be integration with doauto or other analysis to attempt to detect when A,X, or Y is loaded with a variable which corresponds to a subroutine address which has been marked manually.

As an example, running the following the commands:
```
doauto 
domanual 00B19B 000B7D
``` 
on the Anthrox 1992 Christmas Demo creates the following code:
```
NativeNMI: ; $008B68 / $808B68
	php                 	; $B68
	rep	#$30            	; $B69
	pha                 	; $B6B
	phx                 	; $B6C
	phy                 	; $B6D
	phd                 	; $B6E
	phb                 	; $B6F
	sep	#$20            	; $B70
	lda	#$80            	; $B72
	sta	$2100           	; $B74 | $2100 -> $002100
	rep	#$20            	; $B77
	lda	$76             	; $B79 | $76 + d=$0000 -> $000076
	beq	loc_B80         	; $B7B
	jmp	($0076)         	; $B7D | $0076 -> $000076 | pbr = $00
...
loc_17EA: ; $0097EA / $8097EA
	jsr	sub_1B0B        	; $17EA
	jsr	sub_1B51        	; $17ED
	jsr	sub_1AA0        	; $17F0
	jsr	sub_1A32        	; $17F3
	lda	#$2200          	; $17F6
	sta	$7C             	; $17F9 | $7C + d=$0000 -> $00007C
	ldx	#$C073          	; $17FB
	jsr	sub_19CA        	; $17FE
	ldx	#$BB7D          	; $1801
	lda	#$B19B          	; $1804
	sta	$76             	; $1807 | $76 + d=$0000 -> $000076
...
sub_319B: ; $00B19B / $80B19B
	sep	#$20            	; $319B
	rep	#$10            	; $319D
	ldx	#$0000          	; $319F
	stx	$2116           	; $31A2 | $2116 -> $002116
	lda	#$00            	; $31A5
	sta	$2115           	; $31A7 | $2115 -> $002115
	ldx	#$0000          	; $31AA
loc_31AD: ; $00B1AD / $80B1AD
	lda	$048000,x       	; $31AD |  $048000 -> ROM $020000
	sta	$2118           	; $31B1 | $2118 -> $002118
	inx                 	; $31B4
	cpx	#$4000          	; $31B5
	bne	loc_31AD        	; $31B8
	jmp	loc_289C        	; $31BA
```