Provide an API for custom partial resolution

[AndrewLeedham]

Currently in order to implement custom partial resolution, you need to override the function Handlebars.VM.resolvePartial. This seems a bit hacky so perhaps there should be a top-level API to support this.

An example of the current solution:
https://jsfiddle.net/AndrewLeedham/91dacogf/4/

Perhaps the new API could be implemented using the register pattern helpers and partials. So a consumer could provide a function that takes a name and returns a modified version. Perhaps: Handlebars.registerNameResolution((name) => name.replace('@', ''));.

[nknapp]

Looks like this is another case for defining a generic "hooks" api, like in #1435