Filters restructuring

[tomaskir]

This PR contains restructuring of the Filters class.

What was done:

• split built-in filters into a separate BuiltinFilters class
• added a VT100 sequence filter into the new BuiltinFilters class
• split Filters#chain(...) method into a separate FilterChain class

Why: It makes it easier for new users to understand the purpose of each of the new classes. BuiltinFilters class contains built-in filters that can be used out-of-the-box. Fitlers class is now a class which can be used to build new custom filters. FilterChain class is used for building filter-chains, nothing else.

By separating these 3 constructs into separate classes, new users will have an easier time picking up on how filtering works in ExpectIt.

Downside: Doing this breaks compatibility with existing code that was importing Filters class to use the built-in filters. Users will have to change imports to the new BuiltinFilters class. Same with the new FilterChain class.

[agavrilov76]

Thank you for your PR! Since this is non-backward compatible change I cannot include it into a minor version release. Will include it into 1.0.