Option to sync after a specific date or tweet

[Still34]

Summary

At the moment, it seems like once the instance is started, it reads back the past 200 tweets from the user and syncs them across the specified services; this may or may not be desirable to users that wish to only sync posts from now on or after a certain date, i.e., the user wishes to sync posts that they've made after setting up the instance.

[trfetzer]

This is the same suggestion as #218