Value source.

[dbuenzli]

W.r.t. env and cli support it is sometimes annoying not to be able to know where the value was actually specified; and this would only worsen with #35.

Still most of the time you may not care so the current interface seems right, most of the time. For these other cases, we could expose a lower-level converter whose parser type is:

[`Env | `Conf | `Cli ] -> string -> ('a, [`Msg of string]) result 

It would then be trivial to expose higher-order combinators that make use of the current converters that allow to tuple the value with the source e.g.:

val source : 'a Arg.converter -> ('a * [`Default | `Env | `Conf | `Cli]) Arg.converter

But for this cmdliner needs a full internal rewrite.