Add comments

[QuickWrite]

This pull request add the feature that comments can now be added.

The syntax of the comments can be specified through a setting called comment:

local asm = LuASM:new(instructions, {
    comment = ";.*$"
})

This will now allow for comments like this:

start:  mov 10 r0  ; This is a comment
        add  r0 r1
        jmp  start

---

Closes #2

[QuickWrite]

Looks good to me.