Add an -s flag which runs a single hook, and ignore signals

This PR does two thing : 
* Add an -s flag to run a single hook. It just runs the first one it finds.
* Ignores the signal plugin. Currently, If a SIGINT signal is received, pluginhook dies,  and the hooks may continue running if they have ignored the signal. Instead,, just ignore SIGINT, pluginhook quits when the hooks are finished.