This is usecase for #339

Scenario:

• Platform with slow file-system access and a lot of files
• Need initial list of files as well as events on changes

Now I cannot obtain initial list of files from the API. I have to call separately walkdir which imply some drawbacks:

1. I need to rescan the whole file-system again
2. If one file is changed during rescanning, it changes can be lost (initial scan output will be provided later then changed event)
3. It imply memory cost for rescanning large file-system again
4. There may be some platform-specific inconsistencies in case symlink/hardlink. Initial set of files from notify may differ from walkdir output (unless notify is using walkdir internally, I haven't check up that)