Add a cache

[fredszaq]

This will be usefull in the maven plugin for example : if the output dir is in target, a clean means we'll have to regenerate all images. This would affect (badly) next build's build time.

If we add a cache folder outside the target one, then there is no need to regenerate all images, only the ones that have changed.

When using the force update flag, the cache dir has to be wiped out