Support alternate default branches for CI badge

[Daniel Bachhuber]

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

[Wojciech Smoliński]

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

[Daniel Bachhuber]

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