support utf8 regexes

It would be nice to support UTF8 regexes.

Here is an example of doing an UTF8 regex with grep:


Here is what happens when using highlight:


Note that highlight is just working on a character-by-character basis, so it is possible to do a regex on japanese if you account for most japanese characters being 3 bytes in utf8:


When running the three previous examples, my locale settings are as follows:

```sh
$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=
```

Changing it to `LC_ALL=en_US.ASCII` makes `grep` ignore utf8 and output the same thing as `highlight`.