Enhance the manifest by providing file size validation

[robertohuertasm]

It seems that optimization process is not deterministic causing some assets to be changed every time we do an import.

This can be partially solved by using git and detect this kind of things but it's a tedious job to do.

Ideally, the tool should be able to detect changes in file size and let the user know which images changed and how they changed (are they better optimized or not).

In a future feature, it may be nice that the user defines a threshold and depending on that, the tool automatially rolls back some of the changes.