feat: add support for `Provide*() (<type>, error)` methods on commands

fix: regression where all args became passthrough

Fixes #475

feat: add old "passthrough" behaviour back in as an option

`passthrough:""` or `passthrough:"all"` (the default) will pass through
all further arguments including unrecognised flags.

`passthrough:"partial"` will validate flags up until the `--` or the
first positional argument, then pass through all subsequent flags and
arguments.

fix: NPE due to checking if error is nil when it can be a value

Fixes #468

feat: support recursive injection of provider parameters

This allows provider functions to accept parameters that are injected by other
bindings or binding providers, eg. call the provider function with the root CLI
struct (which is automatically bound by Kong):

  kong.BindToProvider(func(cli *CLI) (*Injected, error) { ... })

fix: allow duplicate custom negated flags

Fixes #456

fix!: Include `--` in passthrough args (#436)

Given a grammar like this:

```golang
var cli struct {
        Args []string `arg:"" optional:"" passthrough:""`
}
```

If Kong parses `cli foo -- bar`, it will populate `Args` with `[]string{"foo", "--", "bar"}` (including "`--`").
However, if Kong parses `cli -- foo bar`, will populate `Args` with `[]string{"foo", "bar"}` (leaving off `"--"`).

This differs from the behavior of a passthrough Command, where `"--"` is included with the args in both cases.

There are 3 places where `c.endParsing()` is called
1. When `node.Passthrough` is true: https://github.com/alecthomas/kong/blob/5f9c5cc822bdb888a3671c44d4688a6f602ecb90/context.go#L366-L368
2. When `arg.Passthrough` is true: https://github.com/alecthomas/kong/blob/5f9c5cc822bdb888a3671c44d4688a6f602ecb90/context.go#L451-L453
3. When `"--"` is encountered: https://github.com/alecthomas/kong/blob/5f9c5cc822bdb888a3671c44d4688a6f602ecb90/context.go#L384-L387

The first two do not also pop any tokens. The third one does.

This commit makes `c.scan.Pop()` conditional, skipping it when the next positional argument is passthrough.

I believe this will cause Kong to behave a little more consistently — and from my perspective, `--` is relevant for args intended to be passed through! — but it will change the behavior of existing projects that use `arg:"" passthrough:""`.

Make negatable flag name customisable (#439)

* fix: Check if negatable duplicates another flag

Add a check for flags with the `negatable` option if the negative flag
conflicts with another tag, such as:

    Flag   bool `negatable:""`
    NoFlag bool

The flag `--no-flag` is ambiguous in this scenario.

* feat: Make negatable flag name customisable

Allow a value on the `negatable` tag to specify a flag name to use for
negation instead of using `--no-<flag-name>` as the flag.

e.g.

    Approve bool `default:"true",negatable:"deny"`

This example will allow `--deny` to set the `Approve` field to false.

fix: typo in README (#451)

fix: slices/maps of existingfile would fail to work