Document how to work with custom parsers

[mpscholten]

E.g. when dealing with a custom value object A it might make sense to create your own ->param(..)->a()->required().

We should add documentation on how to do that and maybe change code to make the process easier.

TODO:

• Creating your own AParser
• Extending TypeParser with ->a()
• Configuring the library to use the custom TypeParser

[yuvalherziger]

Nice 👍
We could even add a CLI script that can be run à la:

$ /scripts/generate custom-parser -c AParser
### and also for custom exceptions:
$ /scripts/generate custom-exception -c AException

[mpscholten]

Great suggestion :) Could you open an issue for that?

[apeiniger]

+1 for custom types