Support (?i) flag

[glittershark]

The README says this would be easy to support, but I didn't find an issue for it so I figured I'd open it here

[gfredericks]

well I doubt it would be easy. Just not impossible :)

to do it well would be tricky because the flags can occur anywhere in the regex and have some weird stateful effect of turning things on and off. It would also require figuring out what case-insensitivity means for the entirety of unicode.

[gfredericks]

(never intended to close this)