FR-2 Allow arbitrary (unknown) tokens to be replaced by callback

[niesfisch]

sometimes you have a string with tokens that are not known until runtime.

e.g.

some text with a ${token} and ${anotherOne} and ${anotherOne}.

as a developer i want to be able to register a callback which gets called for every token that is found.

so the callback would be called for

callback.replaceToken("token")
callback.replaceToken("anotherOne")
callback.replaceToken("anotherOne")

and so on

:)