Support case-insensetive matching

[Totktonada]

We can use case-insensetive indexes under hood and it will be work while we really use only the indexes. But once we going to use, say, offset argument a postprocessing of objects may needed, so matching will be case-sensetive. It may be unexpected by an user.

We need to add an option to match case-insensetive or, alternatively, add separate set of arguments (like we doing with PCRE, see #74) or add to filter (which is not implemented yet). Also we can allow to redefine the compare function.

Open question: how to consider locales? I think we can use https://github.com/starwing/luautf8 for that.

[Totktonada]

I think PCRE with (?i) and UTF-8 support close all our needs.