FEATURE: Make formState initialization extendable

[daniellienert]

This adds a mechanism to hook into the form state initialization and define custom initializers.

Resolves: #152

[daniellienert]

On the sprint, we agreed to:

• simplify the function, leave the chaining feature to the custom code
• make the initializer configuration a part of the preset code

[daniellienert]

Closed in favor of #154