[Feature request] Ternary operator

This would be mostly useful for within user-defined functions (#201), but would also be more terse than the current `if/else/endc`.

Example:

```
giveitem: MACRO
db giveitem_command
db \1 ; item
if _NARG == 2
db \2 ; quantity
else
db 1 ; quantity
endc
ENDM

; simplifies to

giveitem: MACRO
db giveitem_command
db \1 ; item
db _NARG == 2 ? \2 : 1 ; quantity
ENDM
```

```
growth_rate: MACRO
; [1]/[2]*n**3 + [3]*n**2 + [4]*n - [5]
dn \1, \2
if \3 & $80 ; signed
db -\3 | $80
else
db \3
endc
db \4, \5
ENDM

; simplifies to

growth_rate: MACRO
; [1]/[2]*n**3 + [3]*n**2 + [4]*n - [5]
dn \1, \2
db \3 & $80 ? -\3 | $80 : \3 ; signed
db \4, \5
ENDM
```