Support configuration struct tags in the documentation generator

[ppar]

The documentation generator at ./docs/generator/ could support the config struct tags introduced in https://github.com/trivago/gollum/pull/143 to:

• Detect undocumented configuration parameters and include them in the documentation
• Detetect and warn about inexistent / mistyped configuration parameters found in the doc comments

Not a very urgent need, but perhaps useful when revising documentation in the future.