Ticket #19877 (New option for management commands to avoid formatting output)

This pull request tries to solve the [ticket #19877](https://code.djangoproject.com/ticket/19877).

I added a `--no-style` option to the `BaseCommand` class, so now if you pass that when executing a Django management command, the output will not be formatted, since it will assign `no_style()` to `self.style` in the `BaseCommand.execute` method.

I've fixed also several regression tests in the `admin_script` module, although I am not sure if something more is needed. If so, please just notify me and I will implement a test for it.

**Update:** I reviewed also the documentation and added explanation for new command option.