Default to nontransitional mode

[indolering]

Are we still in the transitional period?

[jcranmer]

I'm not sure. Firefox has switched to nontransitional. Apparently Safari has well (at least on some platforms). Edge and Chrome appear to be in transitional still.

Chrome's bugs:
https://bugs.chromium.org/p/chromium/issues/detail?id=694157 (nontransitional wholesale)
https://bugs.chromium.org/p/chromium/issues/detail?id=303404 (just switch ZWJ/ZWNJ)
https://bugs.chromium.org/p/chromium/issues/detail?id=303407 (final sigma)

The web platform tests switched to non-transitional, but I don't know if there's going to be a reconsideration if Chrome finds reasons not to switch.

At this point, I'll wait until there's some more clarity from Google and Microsoft on their intentions before switching to nontransitional.

When we do switch, I'll update the version to 2.x to reflect the major change.

[indolering]

I tried updating the README, but its got a lot of contradictory information:

• Is "transitional" mode compatible with all browsers or just Chrome and Edge?
• You state that the library uses transitional mode, not that it defaults to it.
• The API section states that the library defaults to non-transitional mode.

I think it would be best to suggest using non-transitional mode as transitional mode will allow some domains that will be invalid in future browsers. You should probably throw in some language about the major version bump when defaulting to non-transitional mode.