Use the best available reloader when --reload is passed

[berkerpeksag]

Fixes #1422

[mark-adams]

The biggest problem that I see with this change is that the preferred_reloader may not always be desirable. For instance, inotify may be supported by the OS but not by the underlying filesystem. In that case, the library would try to use inotify but would never receive notification of changes because the filesystem driver doesn't support inotify. Currently, the user could simply force polling via the --reload argument and they'd be good to go. With this change, they'd be out of luck and the reloader wouldn't work.