Add one way sync

[jinuxstyle]

Currently, the usage of --two-way option is obvious. User can use it for both uploading and downloading notes. But there is no way for downloading notes only, either a obvious or documented way for uploading notes only. There may be users who want to only download notes to local machine for backup purpose or search useful information from notes more efficiently. This patch adds --one-way option for that. It can be used for either uploading notes only or downloading only.