New feature: Prevent down scaling if there were throttled requests recently

Hi,

I've made some changes to the tool as a measure of precaution for our use case. We have some heavily sharded tables where the ConsumedCapacity metrics don't reflect the actual capacity being used in some of the busier shards. For this reason we wanted to prevent Dynamic DDB from downscaling a table if there were any throttled requests in a certain period of time, ignoring the fact that ConsumedRead/WriteCapacity may be less than or equal to reads/writes_lower_threshold.
I know this is a very specific scenario I'm trying to work around, but if you see any use for this feature/safeguard please consider merging it into the project.

Thanks!
Matías.