chore: optionally allow parsing of hyphen-prefixied flag parameters

This allows for eg. `foo --number -10`, `foo --flag -bar`.

Fixes alecthomas#478, alecthomas#315.

feat: Allow configuring global hooks via Kong's functional options (a…

…lecthomas#511)

Lets you pass `kong.WithBeforeApply` along with a function that supports dynamic bindings to register a `BeforeApply` hook without tying it directly to a node in the schema.

Co-authored-by: Sutina Wipawiwat <swipawiwat@squareup.com>

refactor: ParseError can carry an exit code

Reinstated use of ParseError for all error paths so as to retain existing semantics.

chore: add test for decoding

Load environment variables as a resolver (alecthomas#480)

hooks: Recursively search embedded fields for methods (alecthomas#494)

* hooks: Recursively search embedded fields for methods

Follow up to alecthomas#493 and 840220c

Kong currently supports hooks on embedded fields of a parsed node,
but only at the first level of embedding:

```
type mainCmd struct {
    FooOptions
}

type FooOptions struct {
    BarOptions
}

func (f *FooOptions) BeforeApply() error {
    // this will be called
}

type BarOptions struct {
}

func (b *BarOptions) BeforeApply() error {
    // this will not be called
}
```

This change adds support for hooks to be defined
on embedded fields of embedded fields so that the above
example would work as expected.

Per alecthomas#493, the definition of "embedded" field is adjusted to mean:

- Any anonymous (Go-embedded) field that is exported
- Any non-anonymous field that is tagged with `embed:""`

*Testing*:
Includes a test case for embedding an anonymous field in an `embed:""`
and an `embed:""` field in an anonymous field.

* Use recursion to build up the list of receivers

The 'receivers' parameter helps avoid constant memory allocation
as the backing storage for the slice is reused across recursive calls.

fix(Context.Run): Don't panic on unselected root node (alecthomas#484)

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

fix: regression where all args became passthrough

Fixes alecthomas#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.