Convenience method for recursive directory iteration

[mkruisselbrink]

Copying from #22 point 4:

It should be easy to read only files or subfolders from a parent folder, and have a recursive option to return the contents of subfolders. Example:

try {
 for await(let file_reference of folder_reference.read({ files: true, folders: false, recursive: true })) {
  console.log(file_reference)
 }
} catch(error) {
 console.error(error)
}

[mkruisselbrink]

This would be solved by being able to pass options to getEntries (or to whatever happens with that method). Seems like a nice future enhancement, but probably won't make an initial version.

[mkruisselbrink]

Related to this #154, recursive iteration should somehow make a relative path available for all the returned entries.

[a-sully]

This issue has been ported to the new spec: whatwg/fs#15