FocusLock should have cross-frame behavior controllable

[theKashey]

reach/reach-ui#536

I am adding new crossFrame option, right now defaults to true, as long as this behaviour might introduce a breaking change for someone.

setting crossFrame=true will work "as today". Focus will try to keep focus
setting crossFrame=false will work "as proposed". Iframes would be treated as "pages", and let you move focus outside.
I am not sure which behaviour is actually correct - iframes are usually a part of a page, a part of your product, and the only exemptions are non-production examples like code-sandbox.

[moodysalem]

IMO the default should be false, as it's surprising this behavior is even possible in the browser

[yevhenii-username]

I think this crossFrame option must be described in the documentation.
I faced this issue when tried to use https://github.com/vector-im/element-web inside an iframe and when their modal is open and all my inputs lose focus on the main page.

[yevhenii-username]

Updated readme with crossFrame prop - #220