Add custom sync stategy

[badsyntax]

The current sync strategy is hard-coded and compares contents hash, content-type & cache-control headers to determine whether to sync or not.

Let's make this more flexible and let the user decide how to sync.

Something like:

sync-strategy: |
    # criteria
    etag
    headers.Content-Type
    headers.Cache-Control
    size
    modifiedTime

Not sure on the format, but this shows the intended level of flexibility.

In the above example the criteria are AND'ed. The user can remove or add criteria.