Allow workers to timeout.

[tgxworld]

#21

[tgxworld]

cc @SamSaffron

[tgxworld]

@toy Would you prefer to have the timeout option passed to optimize_image or optimize_image! instead?

Also, I'm having trouble getting the tests to pass on JRuby as it might be related to
jruby/jruby#2684. Would love to have some help here.

[tgxworld]

Hmm I'm trying to think of another way to do this without Timeout.timeout. Will have a think about it.

[toy]

@tgxworld I was late to reply while it was open. Anyway nice and thanks for contribution.
I would prefer a global config exactly as you did. Maybe even better to allow it to be set also per worker somewhat like for allow_lossy, some tools tend to be more time hungry than others, so setting global timeout to for example 30 seconds and timeout for some specific worker to 300 seconds.
Don't forget to add an entry to changelog and add the option to cli options and readme.