Add option to specify basepath

[vitonsky]

Add some option, to serve files with some prefix.

For example, i want to serve directory ./foo with prefix /static. It means that url /static/bar must be resolved to ./foo/bar.

It would be especial useful for developers who want to run simple static server for dev environment, instead of docker with nginx and other infrastructure. The typical problem in this case is that files hosted after some prefix like in example above.