Add setup_swagger(path_prefix) parameter

[asvetlov]

The param should be used as prefix to yaml definitions.

Now swagger_path accepts either absolute path or relative to current directory.

The parameter will allow to use relative to prefix paths.

Moreover, we could add loader concept like Jinja2 loaders -- I believe it makes sense.

[cr0hn]

Use of path_prefix could be good idea, but initially I choose this approach thinking in simplicity.

You're right that in the swagger_path is missing the consideration of relative / absolute paths. I'll take a look.

[asvetlov]

Thinking on simplicity I'm inclining to adding loaders. If no loader is specified by setup_swagger call the path is just a path (relative to current folder or absolute). But if the loader is specified the path is not file path but a parameter for loader. It could be relative path or just a name if dict -- it depends on loader which accepts the parameter. Just like jinja2 does.

We'll keep basic usage simple but allow to use very complex ways for yaml spec finding.

[olexandr-klymenko]

fixed in #34