Forward exitcode after signals like SIGINT

[mrcljx]

HTTP servers tend to support graceful shutdown and will return
an exitcode of 0 when sending a signal like SIGINT.

This change will make hupper return that exitcode instead of
always exiting with 1.

Should fix #80 if the IDE does not break on SystemExit(0).

Test Plan

• Ran hupper --shutdown-interval 5 -m http.server
• Hit Ctrl+C
• Observed exitcode of 0

[mrcljx]

• I tried to ensure not to accidentally revert https://github.com/Pylons/hupper/pull/67/files
• I have not found a good place to add tests for this behavior. Do you have any guidance?
• CONTRIBUTING mentions updating CHANGES.rst, but there is no "next" section at the top, so I didn't feel good about adding a whole section there.

[mmerickel]

Hey thanks for this, I'll try to find some time to look at it in the next few days!