Support for local html file browsing

[magouyaware]

I was really excited when I came across this project, but quickly realized generated HTML files can't be browsed locally. In looking at issues I came across this: https://github.com/SwiftDocOrg/swift-doc/issues/25#issuecomment-609439333

The workaround of running a local web server just won't work for our use case. We are wanting to bundle documentation with a zip of our SDK that will be browsed locally.

For reasons I can't really take the time to go into, we can't host the source code and documentation publicly.

It would be really awesome if a new parameter could be added to provide output specifically for local file browsing: swift doc generate -local -f html