Exit process cleanly on SIGINT, SIGTERM, and SIGUSR2 signals

[carpie]

Hi!

We are using this plugin for SSR workflow to take advantage of cssnext functionality. In our development workflow we run babel-node through nodemon. When nodemon restarts, it uses the SIGUSR2 signal to cause restarts in child processes. This plugin restarts and reports that it cannot create a new server because the existing one is already running and exists. (It seems to be because the socket didn't get cleaned up.) Also, when we exit nodemon, we have to interrupt twice to get the postcss-server process to end.

This PR represents the tweaks I came up with to use this plugin for our workflow. If it works for other work flows, it would be great if it could be merged!

The basic changes are to cause the postcss-server to exit on any of the handled signals and delegate the exit handler to clean up the server socket (since it's called in all cases).

Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling db00bdc on carpie:master into ce92e02 on wbyoung:master.

[coveralls]

Coverage remained the same at 100.0% when pulling db00bdc on carpie:master into ce92e02 on wbyoung:master.