Please add value support for attributes to the customs.json

For ionic for example, there is a attribute called nav-direction. For this there are several values like back, right, left, front and so on. It would be nice if those values can comes up for code completion inside: nav-direction="" so that I see the list of all available values for nav-direction.

For example:

"attributes": {
   values: ["right", "left", "back"]
}

so for this it would be very nice and simple to add code completion for values for attributes in HTML files.

Regards

Chris