choosing varnames in output
(We've talked about this a bit before, but I've found a nice clear use-case for this, and figured I'd make an issue to hold discussion.)

This cart is a simplified version of my pico1k jam entry: 
```lua
-- gitignored/test.lua
x,lvl=8,1
::_::
menuitem(1,"level "..lvl,function(b)
 lvl+=b\2%2-b%2
 menuitem(nil_,"level "..lvl)
end)
b=btnp()
x+=b\2%2-b%2
flip()cls()
spr(0,x*8,64)
goto _
```

Notice that `b\2%2-b%2` is repeated. It would be great for it to be repeated in the output as well, but unfortunately it uses different variable names (`e\2%2-e%2` and `n\2%2-n%2`).

```sh
> shrinko8 gitignored/test.lua out.lua --minify --focus-compressed --no-minify-lines --count
tokens: 60 1%
chars: 145 0%
compressed: 124 1%
```

```lua
-- out.lua
e,l=8,1
::e::
menuitem(1,"level "..l,function(e)
l+=e\2%2-e%2
menuitem(o,"level "..l)
end)
n=btnp()
e+=n\2%2-n%2
flip()cls()
spr(0,e*8,64)
goto e
```

If I manually change the varnames, I can get it down to 120 compressed, but I want the renaming to happen automatically every time I build my game. I _don't_ want shrinko8 to be smarter and do it automatically -- I just want more control over this somehow.

How can I get the out.lua to use the same variable names in the `e\2%2-e%2`/`n\2%2-n%2` lines?