flags: Skip parsing numeric-looking argument with no option

**Is your feature request related to a problem? Please describe.**

When parsing numeric-looking arguments which is not a flag, they are parsed as number. There are cases which it is not desired to do so. For example:

```ts
// $ deno run example.ts 000000 '000000'
import { parse } from "https://deno.land/std@0.144.0/flags/mod.ts";
console.dir(parse(Deno.args));
// output: { _: [ 0, 0 ] }
```

**Describe the solution you'd like**

There is `string` option to indicate argument names to be treated as string instead, but since the above argument is not a flag, it can't be specified with `string` option. It would be nice to have another option to indicate such argument.

**Describe alternatives you've considered**

Setting `-` as `string` works, but it skips number parsing all arguments in `-`. I can't think of another way.