add composer install arguments

This allows to add additional arguments to the `composer install` command.

Example `.ccconfig.yaml`:

```yaml
BaseConfig:
  WebContent: /web
  AdminEmail: mail@example.com
  ComposerInstall: --prefer-source --optimize-autoloader

PHPConfig:
  apcEnabled: 1
  xdebugProfilerEnable: 0
  xdebugDefaultEnable: 1
```