support for variables in message: "just <x> characters to go for the title"

[0xgeert]

Would we great to have support for (mustache) expansions/variables in validation messages to do something like "just <x> characters to go for the title

[platinumazure]

Maybe this feature request could be combined with pull request #199 somehow? Seems like allowing msg to be a function wouldn't be a terrible idea... although in this case the function should be called with something like (value, validator, computed) and the other case would accept a string...

Presumably the framework could be tweaked where if msg was a string, it could turn into _.constant(theString) on the backend and silently discard those arguments, but an actual specified function would have enough information to generate those sorts of messages?

@blikblum @chiefGui Thoughts? If you're interested I could open a pull request to start the discussion, though I could use suggestions on what values should be passed to the message function.

[blikblum]

I think allowing a function with params is the way to go. It will handle the use case of this issue

[platinumazure]

I might open a separate issue just on how "default messages" (even if overridden) are handled differently from what I might call "inline messages" (per-validator). Inline messages can be functions returning strings, but are called with no parameters (via _.result) and this is set to the validator object. Default messages must be strings and have the private format function called by the default validators (supporting {0} style formatting). It would be awesome to get those unified.