Create custom Composer installer for local development

[jmichaelward]

In the milestone/0.2.0, I've temporarily added a post-install-cmd to set configuration on the development instance of PHPCS so my IDE can locate my style rules for the plugin. This functionality is not needed when not doing development and, in fact, Composer will display an error when composer install --no-dev is run, which will be required to properly build a production release of this plugin.

Instead, we'll need to create a custom installer that checks whether the --dev flag was run before setting the installed paths for the local PHPCS. See: https://getcomposer.org/doc/articles/scripts.md, specifically the section that talks about creating the installer in PHP, and the COMPOSER_DEV_MODE value.

This is not a priority for the initial milestone release, but it's something that should be addressed in the future so others and I can continue to develop this plugin following project-specific coding standards instead of global ones.