Allow additional custom prettifiers in the app config

[hollodotme]

Currently all prettifiers are enabled by default and must be part of the application.

Find a way to add custom prettifiers via app config.

Things to consider:

• Autoloading of custom prettifiers
• Additional parameters for custom prettifiers

For reference, see #16 / cc @Fishdrowned

[Fishdrowned]

For autoloading I have 2 options:

1. Add a new directory something like plugins; or
2. Use wikimedia/composer-merge-plugin to load custom composer packages

[Fishdrowned]

I don't have idea which additional parameters are essential, I prefer to remain the interface PrettifiesString unchanged.

[hollodotme]

@Fishdrowned Thanks for the pointers and explanation in #16.

With additional arguments I meant optional constructor arguments for the custom prettifier class, like e.g. when you write a PHPUnit test listener. The constructor is not part of the PrettifiesString interface, so it would remain unchanged.

If nothing gets in my way, I'll have time to work on this tomorrow evening (CEST). 🤞

[hollodotme]

@Fishdrowned I have a working prototype using wikimedia/composer-merge-plugin and hollodotme/readis-base64img-prettifier as a custom prettifier. Build is currently failing, so I need to fix that and also add some tests. More details soon...