Add an option for sending SIGINT instead of SIGKILL

[canthefason]

With each file change, we are sending SIGKILL command for killing the running process. The problem is, it would leave some open connections behind. For the programs that require graceful shutdowns, it is better to have an optional SIGINT support for go-watcher.

[julianxhokaxhiu]

CLI argument for the binary, ENV variable for the Docker ( which will be then passed as CLI argument ), is my proposal. What do you think?

[penkovski]

@canthefason Do you plan to add this feature or you prefer contribution for it?