Added a skip-subreddits flag

[woodgun]

Added a new flag skip-subreddits allowing users to skip deleting any of the "things" from the given subreddits. Example: shreddit --skip-subreddits rust,swift

Note, I didn't have a chance to test with GDPR data (just requested it) so I assumed the model would be the same (would have subreddit param).

[andrewbanchich]

awesome, thanks for the contribution!