Add unicode support for strong/em regex.

Hi, do you think maybe it would be reasonable to add support for unicode characters in regexes like `$em_regex`, `$strong_regex` etc.?

For example, let's take a filename: `black_cat_picture.jpg`.
With github flavored Markdown we can see underscores in the middle of the word: "black_cat_picture.jpg".

Now, let's try the same thing with Japanese filename `黒_ネコ_の絵.jpg`.
In this case Markdown will return: "黒_ネコ_の絵.jpg" (ネコ is being italicized).