Add option to disable closing issues

[goanpeca]

Great project :-)!

Having a bot close issues is considered to be very rude under some circumstances so it would be great to be able to decide if this action does not close issues at all.

Something like days-before-close: -1 should serve as a flag for this and let the project maintainers/authors do any actual closing.

A workaround could be to add a very very long time to days before closing, but that does not seem like the cleanest solution.

[goanpeca]

Oops, I did not see #15 !

Closing!