Add FORCE_START option

This is related to the issue #12. 

Added `FORCE_START` option so that fluent-agent-lite (tail -F) will start even if the target specified file does not exist. This option enables us to deploy `fluent-agent-lite` before application outputs logs, I mean, before service-in. 


HOW TO USE: In fluent-agent-lite.conf, add

    FORCE_START=1

Then, the init script runs fluent-agent-lite with `-F` option.