Async client side custom validators in Epiforms

Custom validators for Epiforms must currently return their validation result synchronously, this limits the kind of validation to simple string checking.

In scenarios where a server must be consulted to check an input value it is often a better experience for users to get feedback before they attempt to submit the whole form.

There is an example in the Samples of a synchronous ajax call in a validator. This is potentially very bad for user experience. While we wait for the ajax to return the browser will be non-responsive.

Something based on promises would be most welcome.