Command to run on whole project

Although currently we run only on 1 file, we should give the user an option to run coala on the whole project.

Now, the question is how to get the project ? In sublime, the project is a list of folders.

So,
1. We could find the coafile for each folder and run them. If not found, ignore it.
2. We could ask the user to select which folders to run coala on (along with a command for "all")
