Allow scheduling updates for specific dependencies

[bhearsum]

Generally, I prefer to receive a PR for every update, and as soon as possible. This makes it easier to stay up-to-date, and to hold back specific upgrades that can't be merged. Pyup works great for this, but it becomes painful and spammy when you depend on a package that follows a continuous deployment model, and releases multiple times a day.

For these cases, it would be helpful to be able to schedule that specific package to upgrade on a certain day, or to limit it from being updated more than once per X (eg: day).