Allow customizing hint in required

[dhoepelman]

Fixes #36

Th syntax is (also seen in README is)

Class:prop required {
  hint = "..."
}

this was not my preferred syntax, but it was one of the only viable ones.

Alternative 2:

Class:prop required(hint = "...") { }

is not valid kotlin syntax, infix functions cannot have extra parameters

Alternative 3:

Class:prop required { } hint("...")

would require changing the return type of these validationbuilder from Unit to something else, most likely Validation, and adding hint to ever validation, which I don't want. I couldn't think of a good alternative for returning.