Allow default commands with cmds/args/flags (#188)

* Move default command validation to build

Move the validation of default commands - checking if a node has
multiple default commands or a default command has children - to the
build phase rather than tracing. These errors are with the structure of
the CLI ast and are detectable before parsing the command line.

Add a couple of tests for some of the default command error cases.

* Disallow positional args on a default command

Do not allow a default command to have positional arguments. The current
check is only for branching args, but the error message implies that
positional args are not allowed either. So add a check for positional
args too, and add a test case for it.

This is breaking change to the API but is unlikely to have ill-effect as
positional args on a default command cannot be used without explicitly
naming the command (i.e. not using it as a default).

* Allow default commands with cmds/args/flags

Allow default commands to have sub-commands, args and flags when tagged
with `default:"withargs"`. This makes specifying the name of the
command on the CLI completely optional as long as the args to that
command are not ambiguous with other commands.

model.go

@@ -51,6 +51,7 @@ type Node struct {
 	Flags      []*Flag
 	Positional []*Positional
 	Children   []*Node
+	DefaultCmd *Node
 	Target     reflect.Value // Pointer to the value in the grammar that this Node is associated with.
 	Tag        *Tag
 	Aliases    []string