Cannot use '%' or '#' in cmd arguments

Commit b62de9d introduced the nice feature of having % and # automatically expanded. Unfortunately, it makes it impossible to e.g. `:Grep asyncdo#run` (grep for "asyncdo#run" using the `:Grep` command suggested in the docs).

I'm not exactly sure what the best way to handle this situation is, but I feel like % and # substitutions should be more explicit. What about some markers that represent explicit expansion? e.g.:

```vim
" From autoload/asyncdo.vim, expands to:
" :AsyncDo grep asyncdo#run autoload/*
:AsyncDo grep asyncdo#run {{%:h}}/*
```

This makes the expand substitution simpler (just look for `{{...}}`), and more powerful as it doesn't need to specify which things get expanded (currently `%` and `#` with `phrte` filename-modifiers, but not other filename-modifiers or expandable settings).

I can make a PR if this sounds like a sensible approach?