Add a new I18n.isolation_scope= option to explicitly use threads or fibers

[travisbell]

After the update on #723 (#724) I18n became incompatible with web servers like Falcon, that serve every request in a new fiber. You can read my comment on that issue for a bit more context.

This PR introduces a new option called I18n.isolation_scope which allows you to configure where you'd like the instance of I18n::Config to be stored. By default it stays as a thread variable, but it can be changed to use fibers like so:

I18n.isolation_scope = :fiber

By using :fiber we store the config in a fiber local Fiber[:i18n_config] instead of thread local Thread.current.thread_variable_get.

[travisbell]

P.S. I looked into using the value from ActiveSupport::IsolatedExecutionState.isolation_level but since ActiveSupport is not a hard requirement for I18n, I don't think it made sense. For any apps using fibers like this, they'll just need to set both values. That is, unless we add some complexity to the check.