Log: defined with -nargs=+, but handles only one

Defined in https://github.com/junegunn/vader.vim/blob/85a04a53857453b038fb89648622d85b48da60fd/autoload/vader.vim#L171, but only one arg is handled:

``` vim
function! vader#log(msg)
let msg = type(a:msg) == 1 ? a:msg : string(a:msg)
call vader#window#append('> ' . msg, s:indent)
endfunction
```

I have tried `Log foo bar` several times, resulting in an error.

I think it should be `vader#log(...)`, looping over `a:000`, and add the result joined by commas?! (so each `Log` call is a single line still)