add option: create non-existing files before updating existing files

[bojieli]

Rsync does not provide atomicity, so, the links in existing pages (e.g., homepage) may be updated before new large files (e.g., images and media) are uploaded, leading to dead links.

This patch introduces a 'create_before_update' option to upload non-existing files before updating links to them in the existing pages.

[stevenjoezhang]

There are some eslint errors need to be fixed.

[bojieli]

There are some eslint errors need to be fixed.

Fixed, thanks!