Change type of OUTPUT_RETENTION from tuple to list.

Currently, the `OUTPUT_RETENTION` setting is described in the documentation as being a tuple. However, it seems realistic to accidentally define the setting as `OUTPUT_RETENTION = ('.git')` instead of `OUTPUT_RETENTION = ('.git',)`, which is a string rather than a tuple. The [relevant code](https://github.com/getpelican/pelican/blob/master/pelican/utils.py#L295) would then fail to retain the file/directory.

Since the `DELETE_OUTPUT_DIRECTORY` setting is destructive, it seems to be a safer option to define the setting as a list rather than a tuple to prevent this mistake from happening.