Add cacheDir setting in order to choose which folder use to store cache files

[dabit1]

In order to make easier our life when we have to configure a CI tool with autodll-webpack-plugin is very useful if we can choose the folder where cache files are going to be stored.

The main problem I had was that when TravisCI create a new build it does not have any cache files so webpack is even slower than without using autodll-webpack-plugin. What I did was to copy files from cache directory to another one outside of node_modules and then configure TravisCI to cache this directory. I could not cache the node_modules cache dir directly because when TravisCI execute npm ci the folder node_modules is deleted before installing dependencies.

[ialpert]

Great work, @dabit1 !
This would be extremely useful, any forecast on merge?