Proposition - introduce a new way to specify "per type" flags

[apawelec]

Issue with propagateInitFuncReturns flag (with multiple types) shows that current way of configuration isn't very flexible. I was going to add PR with new flag private (that generates constructors with package-private visibility, which I need) and faced the similar problem. Should this flag affect all types? - it may not be desirable, but looks like only way to do it currently (without introducing quite complex syntax like -private NameOfPrivateType1,NameOfPrivateType2 and so on). I realize that I can have multiple go:generate comments for each type, with separate sets of flags, by that way each constructor has to be in separate file - and I prefer to have constructors for all types from single file to be generated in single file, too.

I have proposition to introduce new way to configure type-specific options, by read it from comments associated with structs in AST (like many similar projects do). It may looks like this:

//go:generate go tool gonstructor -type Foo -type Bar -output cons.go

//gonstructor:flags -init initFunc -propagateInitFuncReturns -withGetter
type Foo struct {}

//gonstructor:flags -constructorTypes builder -setterPrefix With -returnValue
type Bar struct {}

We can leave global flags to treat them as default ones and maintain backward compatibility. With that syntax we can even use flag package to parse them, just like global ones.