Created types for functions

The function used by BashComplete, Before, After, Action and
CommandNotFound have their won type.

This makes easier to change/update the API

funcs.go

@@ -0,0 +1,18 @@
+package cli
+
+// An action to execute when the bash-completion flag is set
+type BashCompleteFn func(*Context)
+
+// An action to execute before any subcommands are run, but after the context is ready
+// If a non-nil error is returned, no subcommands are run
+type BeforeFn func(*Context) error
+
+// An action to execute after any subcommands are run, but after the subcommand has finished
+// It is run even if Action() panics
+type AfterFn func(*Context) error
+
+// The action to execute when no subcommands are specified
+type ActionFn func(*Context)
+
+// Execute this function if the proper command cannot be found
+type CommandNotFoundFn func(*Context, string)