replace the regex sequence w with [[:word:]] to be more flexible

Make liquid more flexible in internationalized situations.

Before:
Markup was only rendered properly if the markup contained latin characters. 
i.e.  {{test}}

After:
Markup can contain non-latin characters, for example Japanese. 
i.e. {{テスト}}

@boourns, @fw42, @camilo, @dylanahsmith, or @arthurnn can you code review?