Exit on end-of-file

It's a common practice shells to exit on end-of-file. WP-CLI doesn't do
this.

I traced the reason to the fact we're not checking the exit code from
the `read` builtin. If it timeouts or receives end-of-file, it's
non-zero. In both these cases it makes sense to exit.
Merge pull request #304 from nb/exit-on-eof

Exit on end-of-file