not_in, nil handling, improved README

Breaking Changes

• Handle null values for in and not_in operators to be aligned with eq.
• like, ilike, starts_with operators now escape arg s.t. % and _ characters are not interpreted as placeholders anymore.
• Field names are now converted to atoms using String.to_existing_atom/1, failing for unknown field names.

Added

• not_eq operator
• ends_with operator

Deprecated

• contains operator for string values: Use like instead, as contains will only be applicable to collections in the future.

Changed

• PredicateError's message now uses inspect on the offending predicate instead of JSON serialization to reduce library dependencies