Allow the user to skip posts that have already been imported

[devilgate]

This allows you to make changes to your local version of an imported post, and not have it overwritten the next time the plugin runs.

To use it, in FEEDS in conf.py, set start_at to now. Here is an example:

FEEDS = {
    'letterboxd': {
        'url': 'https://letterboxd.com/devilgate/rss',
        'template': 'letterboxd.tmpl',
        'output_folder': 'posts/letterboxd',
        'format': 'html',
        'lang': 'en',
        'tags': 'films, letterboxd',
        'start_at': 'now',
        'metadata': {
            'title': 'title',
            'date': ['letterboxd:watchedDate', 'user_date_added', 'published'],
        }
    }

[ralsina]

The failures are unrelated, so I am merging anyway