Support baseDir as CLIs first argument

I would like for it to be easier to set the baseDir at runtime without having to remember the exact flag name and casing of `--baseDir`.

Usage would be:

$ lite-server dist

and it would behave the same as:

$ lite-server --buildDir="dist"

I'm looking at lite-server to replace http-server for the better SPA support but the interface is less convenient.

I'd be happy to work on an initial implementation.