Load guidance limits from JSON file

[s-l-teichmann]

Add a way to load size guidance from JSON files. First step towards #3

[bernhard-herzog]

I've looked at how one might use these when generating. It's easy to
keep track of where the generator is by passing along a slice of strings
that the generator keeps appending to when descending a level. For
properties it adds the property name, for arrays e.g. a string with
"[]". This way you get arrays like this:

"product_tree", "branches", "[]", "branches", "[]", "product"

now we only need to map that to the corresponding limit, if one exists.
I don't quite see how that easily maps to the LenghtPaths. Two
possibilities:

• A mapping from a string representation of the above, i.e.
  "product_tree/branches/[]/branches/[]/product", to the
  corresponding limit would be more helpful.

• An alternative could be a nested mapping where the first level would
  contain the keys document, product_tree, vulnerabilities each
  which maps to a pair of another such mapping, maybe empty, and a
  limit, possibly null to indicate no limit. The generator could take
  this mapping and at during descent extract the appropriate
  sub-map/limit instead of building up a slice of strings.

That being said, I don't mind if we merge this PR and develop it
further.