refactor(commands): separate commands and its core into two folders (#32)

- I think it would be better if one for commands only, one for the spec
handling core logic so people won't be confused by the core logic inside
the commands folder (cleaner I guess)
- Regen docs

spec/spec.go

@@ -0,0 +1,53 @@
+package spec
+
+type GeneratorFunc func(tokens []string, prefix string, partial string) []Suggestion
+
+// Spec defines a top-level command structure
+type Spec struct {
+	Name        string
+	Aliases     []string
+	Description string
+	Icon        string
+	Subcommands []Subcommand
+	Options     []Option
+	Generator   GeneratorFunc
+	MaxArgs     int
+}
+
+// Subcommand defines nested command logic
+type Subcommand struct {
+	Name        string
+	Aliases     []string
+	Description string
+	Icon        string
+	Subcommands []Subcommand
+	Options     []Option
+	Generator   GeneratorFunc
+	MaxArgs     int
+}
+
+// Option represents a command flag or option
+type Option struct {
+	Name        string
+	Description string
+}
+
+// Suggestion represents an item in the suggestion menu
+type Suggestion struct {
+	Cmd  string
+	Desc string
+	Icon string
+}
+
+var Registry = map[string]*Spec{}
+
+// Register adds a new spec to the global Registry
+// example: Register(&Spec{Name: "git"})
+func Register(s *Spec) {
+	Registry[s.Name] = s
+}
+
+// ResetRegistry clears all registered specs - use in tests only
+func ResetRegistry() {
+	Registry = make(map[string]*Spec)
+}