Fix validation of 'git' and 'files' options

[MontealegreLuis]

I found an issue when running the analyzer the following way:

$ bin/phpqa analyze

I got a warning Undefined offset: 0, due to the logic around explode for the files option, followed by the expected exception You must set 'files' or 'git' options..

I moved the validation and normalizing of the files option to its own class with its corresponding tests.

I also added integration tests for the AnalyzeCommand that verify the git and files validation logic I changed.

One thing I noticed is that currently it is not possible to write tests for the command without modifying your src or whatever folder you pass to the test. It might be a good idea to create Analyzer objects and inject them to the app in order to make them testable or create a folder for testing with fixture files.