Implement positional flags support

[rpunkfu]

Hi, this is a feature I've really wanted, so I decided to implement it over the last two weekends whenever I had some free time.
I hope it meets your standards and we can get it merged swiftly! :)

I've added an entry to README.md explaining the usage, so I'm just going to refer to it below, instead of repeating myself.

Recipe Flags

In addition to parameters, recipes may have flags.
Flags are named optional arguments that use --flag syntax and can appear anywhere in the argument list:

build target --release --jobs='4':
  cargo build {{ if release == "true" { "--release" } else { "" } }} --jobs={{jobs}} --target={{target}}

Flags come in two forms:

Switch flags are boolean flags that don't take a value.
When present on the command line, they're set to "true", otherwise "false":

deploy --dry-run:
  @echo "Dry run: {{dry-run}}"

$ just deploy --dry-run
Dry run: true
$ just deploy
Dry run: false

Value flags take a value and must have a default, specified with =:

serve --port='8080':
  python -m http.server {{port}}

$ just serve --port=3000
python -m http.server 3000
$ just serve
python -m http.server 8080

Flags can be passed in any order and can be mixed with positional parameters:

$ just build x86_64-linux --release --jobs=8
$ just build x86_64-linux --jobs=8 --release

Flag names are converted to variable names by preserving the exact characters,
allowing hyphens in variable names when accessed via {{flag-name}}:

test --dry-run:
  @echo "Dry run mode: {{dry-run}}"

To pass a value that looks like a flag as a positional argument, use -- to stop flag parsing:

$ just cmd -- --not-a-flag

---

Fixes: #476

[rpunkfu]

@casey Any chance to get some feedback on this?

[casey]

Hey, sorry to close this, but I wound up implementing it myself in #3026.

I left a comment in another PR which I also shamefully did not review and wound up closing with details on the implementation, and why I went a different way: #2806 (comment)