Adding ability to have a manifests cache

By using a manifests cache, we can prevent the startup time from having to perform another Glob every time. This saves 5-6 seconds on EVERY startup. To clear the cache, simply remove the cache file.

@intervalia / @redbugz, any thoughts?
I assumed a relative path with a "./" which this replaces. Not technically necessary, though. I guess it makes more sense to let the consumer do that.