feat: Implement flag argument parsing

This covers the simplest kind of arguments that could be parsed: switch flags
that could be either `true` or `false`.

Switch flags can only be switched to `true` from the command line, their
default value is falsy (`undefined`).

Flags are also identified by a single lowercase or uppercase letter, and
uppercase flags are different from lowercase flags using the same letter.

To turn a flag on, simply pass its name preceded by a dash, like this (for the
"f" flag):

```bash
$ call program -f
```

To turn on multiple switch flags at once, one can either pass them as
individual arguments or group them up rightafter the dash, so the following
two calls are functionally identical:

```bash
$ call program -a -b -C

$ call program -abC
```