Force extension with --use-ext

[jleonardolemos]

Closes #22

Add an --use-ext option in order to provide a way to force a specific zip tool as the follow:

php security-checker security:check /path/to/composer.lock --use-ext system-unzip

We can use system-unzip force system unzip command and zip-extension to force PHP Zip Extension and any other option will be silently ignored.

Some considerations:

1. If we chose for zip-extension explicitly and the zip extension is not installed than an exception will be thrown, should it be handled?
2. If we chose for system-unzip explicitly and the unzip command does not exists than an error will happen, should it be handled?
3. Because the classes are very coupled with the new operator i was not able to test very well without very clumsy mock strategies, i just wrote a test to ensure that ti code is running fine and nothing is broken.

[paras-malhotra]

Thanks @jleonardolemos, appreciate your contributions 👍 I'll try and get this in by the end of the week. Been busy with some stuff lately.