Support an output folder for formatted reports

[csharpfritz]

When a formatter is applied that is not the console output, Let's specify a default name and allow the user to supply an output folder to write that report to.

This argument should be -o or --output-dir

Originally proposed in #27

[paddybhoy66]

it would be useful if the output documents were automatically saved into say, a docs folder within the project under scan, thus becoming part of the project's documentation.