Add option to avoid making directories for post permalinks that don't end in .html

The current behavior is that if a post's permalink doesn't end in .html, a directory is made with the permalink path and the post is put into index.html in that directory. This is undesirable if the web server is already set up to redirect e.g. /foo to /foo.html.

So I added the option use_post_dirs to control this behavior. If true, it does the current behavior, but if false, it simply appends .html to the permalink path.

Also modified the serve command to emulate a web server that does the /foo -> /foo.html redirection when necessary.

Let me know if there is another better way to do this!