Add 'sync' option to FileOutputter

Unicorn allows to reopen log file when it receives the USR1 signal. This is useful when using logrotate for example.

But to be considered as a log file candidate to be reopened, Unicorn select only synced files. To be able to use this mechanism, this patch add a 'sync' boolean option to the FileOutputter to force the created log file to be synced.