Make Puppet Strings available via PDK

[jstraw]

PDK allows validating and unit testing puppet tests and code, but not documentation. Adding an interface to puppet strings that allows both the generation and validation of puppet documentation would greatly simplify the process of writing and maintaining modules both internally and as a community.

I see a pair of spaces that it would be useful to tie strings into:

pdk validate
Running puppet strings generate with output set to none, so the errors/warnings show but the contents aren't generated

pdk strings
This is basically an alias to puppet strings, so that someone can run pdk strings generate to create a REFERENCE.md or to write html docs that can be posted to the github-pages branch of a module.

These two options would allow for testing strings as part of a local test/CI test and then generating the documentation as part of a release workflow/deployment

I'd settle initially for the current litmus treatment, where puppet-strings was available in bundle (at various puppet versions) so that pdk bundle exec puppet strings worked