Expand directory path when reading an .ini file

In our team we all keep the code under `~/Code` but using the `~` in the invoker _ini_ file is impossible.

Since invoker uses `File.directory?` to check if the `directory` configuration is a valid configuration the path is not expanded and it will result in an `Invoker::Errors::InvalidConfig` exception.

This pull requests adds the path expansion that lets the file specify `~` prepended paths. If needed it should be easy to add the ability to use environment variables too. I intentionally left out that part since it requires a little more work (and I don't think it would be that useful).