Make all classes inheritable. [FEATURE]

[Crusader4Christ]

Parsing or Formatting?

• [x ] Formatting
• [x ] Parsing

Is your feature request related to a problem? Please describe. I'm always frustrated when classes are not inheritable. I need to change some small behavior, but all your classes use private methods, so I can't override them.

Describe the solution you'd like Change all private methods and properties to protected.

Example PR for formatter https://github.com/C2FO/fast-csv/pull/655