v3.0.0rc1

Main changes

• Added a plugin mechanism, allowing users to define and integrate their own "business logic" fields.
• Change the global exception handling error level, from "error" to "exception". It'll provide better insights if you're using Logmatic or any other logging aggregator (#336).

warning: The plugin mechanism is on BETA mode, and at the moment, it only allows to define business-logic fields at "form building time". It means there's not business-logic validation mechanism when you're filling a form previously defined with your custom field.
This is a work in progress, please bear with us.

Minor changes

• Upgrade to Circle-CI 2 (before the end of life of Circle-CI v1 on August, 31st 2018). (#342)
• Optimize Circle-CI usage by using the tox matrix in tests (#343)
• Skip tox installation in the circle-ci environment: it's already there (#344).