Hide sub-tree when parent is hidden.

Ensure flag defaults (or envars) aren't scanned in as untyped tokens.

This avoids treating default values starting with "-" as flags.

Make ExpandPath public.

Don't show ... in help for named slice types.

Expand ~ for FileContentFlag.

Ensure context bindings are used for Run().

Support for adding bindings to the Context.

This is very useful for hooks to pre-construct objects that can be used
by all subsequent downstream commands, for example.

Apply hooks to default values.

Add an "embed" tag.

Add support for setting variables via tag.

This provides much more convenient composition when reusing structs in
different parts of the command grammar.

eg.

	type Embedded struct {
		Key string `help:"A key from ${where}."`
	}

	var cli struct {
		Embedded `set:"where=somewhere"`
	}