Multiple files mode should gracefully handle file paths that are directories

[j13k]

Currently an ugly error is thrown:

 ./bin/yaml-lint ~/foo/ 
PHP Notice:  file_get_contents(): read of 8192 bytes failed with errno=21 Is a directory in yaml-lint/src/yaml-lint.php on line 87

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.