Custom functions

How about custom functions support (or plugins)
Like:
```
seq 5 | filter is_odd
```
will return:
```
1
3
5
```
where `is_odd` is custom function (e.g. in `~/.config/shell-functools/plugins/main.py` directory)
```
@functools.func("Int", "Bool") #or @functools.func(int, bool)
def is_odd(a: int) -> bool:
    return a % 2 == 1
```

but, yes, it looks very complicated...