Fix Unicode regular expressions

[sutherland]

Slugifying strings containing Unicode characters can lead to invalid results:

iex(1)> Slugger.slugify("abc 😀")
<<97, 98, 99, 45, 159, 152, 128>>

Regular expressions do not match Unicode by default. Adding the unicode modifier seems to resolve the issue.

[etiennealbert]

+1 for the merge.
@sutherland you should use ~r/([^a-z0-9\p{Hangul}])+/u to correctly handle the Korean alphabet.

[inhji]

I would love to see this merged. It causes Postgrex to fail with

** (Postgrex.Error) ERROR 22021 (character_not_in_repertoire) invalid byte sequence for encoding "UTF8": 0x9f