Add caching

[pschichtel]

This is especially useful for the hiera/data use-case, where there may be a lot of lookup calls which currently will all lead to completely rebuild of the same model over and over again.
The caching could be as simple as as writing out the resulting file and passing it through if it exists. The obvious cache-key is the node.

[pschichtel]

Caching would be enabled by simply providing a directory to save the cached models to. Additionally there could be an environment variable to force a cache-bypass, which could be useful to temporarily disable caching during environment deployments for consistency reasons.

[pschichtel]

Use this to deliver a file from cache: https://github.com/tokuhirom/golang-samples/blob/master/samples/coreutils/cat.go

...obviously combined with some locking