only display app version and help message once

When processing the flags for -h/--help and -v/--version, only check
the flags in checkVersion() and checkHelp() instead of also printing
the associated message.

This fixes the problem of `app -h` and `app -v` printing their output
twice. The doubling was caused by printing the message once for each
registred alias for the given flags (-h/--help and -v/--version).

Resolves #285

app.go

@@ -138,10 +138,12 @@ func (a *App) Run(arguments []string) (err error) {
 	}
 
 	if !a.HideHelp && checkHelp(context) {
+		ShowAppHelp(context)
 		return nil
 	}
 
 	if !a.HideVersion && checkVersion(context) {
+		ShowVersion(context)
 		return nil
 	}