Support alternate default branches for CI badge

[danielbachhuber]

CI badge generation is currently hardcoded to master:

scaffold-package-command/src/ScaffoldPackageCommand.php

Lines 258 to 263 in f5684be

	if ( file_exists( $package_dir . '/.travis.yml' ) ) {
	$shields[] = "[](https://travis-ci.org/{$readme_args['package_name']})";
	}
	if ( file_exists( $package_dir . '/circle.yml' ) ) {
	$shields[] = "[](https://circleci.com/gh/{$readme_args['package_name']}/tree/master)";
	}

This repo now uses main. trunk is also common.

Done is:

• There's a flag to manually specify the preferred 'main' branch for the package.

[wojsmol]

@danielbachhuber Detecting for all setups wee be hard but we can add a parameter and set default value to for example main.

[danielbachhuber]

@wojsmol I'm fine with skipping the "default branch detection".