Add FORCE_START option

Fix #12. 

1. If one of specified log files does not exist, the init script fails and will not start any of fluent-agent-lite processes now (Default)
2. Added `FORCE_START` option so that fluent-agent-lite (tail -F) can start even if the specified log file does not exist.

This option enables us to deploy `fluent-agent-lite` before application outputs logs, I mean, before service-in. At our Ops team, this is mandatory. 

HOW TO USE: In fluent-agent-lite.conf, add

    FORCE_START=1

Then, the init script runs fluent-agent-lite with `-F` option.