Refactor to renderless (breaking changes)

This release was a major refactor, and a big step toward a stable release. The biggest new feature is the ability to completely replace the template of the query builder, to provide better support for folks who need to integrate with other CSS frameworks or just want more control over the display of the query builder.

Breaking changes:

• selectedOperator and selectedOperand in the query input/output were changed to operator and operand.
• styled prop was removed
• default styling uses Bootstrap 4 classes, rather than Bootstrap 3

See the documentation for more information on providing a custom template.